Immunoglobulin single variable domains targeting pd-l1

Abstract

The present technology provides polypeptides that binds to PD-L1. The present technology also provides nucleic acids, vectors and compositions.

Description

[0001]IMMUNOGLOBULIN SINGLE VARIABLE DOMAINS TARGETING PD-L1 DESCRIPTION 1 Field of the present technology The present technology relates to immunoglobulin single variable domains (ISVDs) targeting PD-L1 and more in particular to polypeptides and constructs that comprise or essentially consist of one or more such ISVDs. It also relates to nucleic acid molecules encoding the ISVDs and polypeptides and vectors comprising the nucleic acids, and to compositions comprising the ISVD, polypeptide, nucleic acid or vector. The present technology further relates to these products for use in a method of treating a subject suffering from cancer. The present technology also relates to methods of producing these products. 2 Technological Background The PD-1 (programmed cell death-1) receptor (CD279) is expressed on the surface of activated T cells. Its ligands, PD-L1 (B7-H1; CD274) and PD-L2 (B7-DC; CD273), are transmembrane proteins belonging to the B7 family commonly expressed on dendritic cells and macrophages. PD-1 and PD- L1 / PD-L2 belong to the family of immune checkpoint proteins that act as co-inhibitory factors, which can halt or limit the development of the T cell response. PD-1 / PD-L1 interaction ensures that the immune system is activated only at the appropriate time to minimize the possibility of chronic autoimmune inflammation. The PD-L1 biology and its role in cancer immunology and immunotherapy are described by Sun et al.2018 (Immunity 48: 434) and Salmaninejad et al.2019 (J. Cell Physiol.234: 16824). A number of biological compounds aiming at PD-L1 blockade have already been approved for use in cancer. Atezolizumab (Tecentriq; Roche), a Fc modified, humanized IgG1 monoclonal antibody, has been approved for urothelial carcinoma, non-small cell lung cancer, small cell lung cancer, hepatocellular carcinoma, melanoma, alveolar soft part sarcoma, and triple-negative breast cancer. Durvalumab (Imfinzi; MedImmune / Astrazeneca), a humanised IgG1kappa monoclonal antibody has been approved for certain types of bladder, lung, and biliary tract cancer. BMS-936559 / MDX-1105 (Medarex / BMS) is a humanized IgG4 monoclonal antibody. Avelumab (Bavencio; Merck KGaA / Pfizer), a fully human monoclonal antibody, has been approved for treatment of Merkel cell carincoma, urothelial carcinoma and renal cell carcinoma. Shuguang et al. 2018 (Protein Cell, 9:135) describes a comparative study of the binding characteristics of these anti-PD-L1 monoclonal antibodies elucidating the rules of binding properties of anti-PD-L1 biologics. The less efficient penetration of monoclonal antibodies because of their large size and the complexity of the tumor microenvironment has impelled the development of small-molecule biologics with superior penetration efficiencies. Envafolimab (KN035; 3D Medicines / Alphamab Oncology / Simcere Pharmaceutical Group) is a single domain-Fc fusion protein and is currently marketed for the treatment of advanced biliary cancer and soft tissue sarcoma. Other immunoglobulin single variable domains that target PD-L1 have been described in WO 2008 / 071447. There is still a need for improved small-molecule biologics to PD-L1. Such small-molecule biologics are also useful for combining with other (cancer)target binders in a single chain, while maintaining a relatively small size, high affinity, potency and / or efficacy, in addition to good manufacturing yields. Furthermore, the biological drug should show safety, convenience, patient compliance and improve patients’ quality of life. 3 Summary of the present technology In some embodiments, the present technology relates to immunoglobulin single variable domains specifically targeting PD-L1. In some embodiments, the present technology relates to high affinity immunoglobulin single variable domains specifically targeting PD-L1. In some embodiments, the present technology relates to high potency immunoglobulin single variable domains specifically targeting PD-L1. In some embodiments, the present technology relates to high efficacy immunoglobulin single variable domains specifically targeting PD-L1. Targeting PD-L1 with a biologic that is small (in size), such as a monovalent or bivalent immunoglobulin single variable domain, and keeping a high affinity, potency and / or efficacy may be advantageous in certain applications where a very small binder is desired. Small binders may be advantageous, e.g., for penetration in the tumor environment, or when additional binders for other (tumor) targets are implicated in the same construct, facilitating manufacturing and further handling of the therapeutic compound. The present technology provides immunoglobulin single variable domains specifically targeting PD- L1 that block the interaction of PD1 and PD-L1 (with comparable or better IC50 than benchmark(s)). The present technology provides immunoglobulin single variable domains specifically targeting PD- L1 that block the interaction of CD80 and PD-L1 (with comparable or better IC50 than benchmark(s)). The present technology provides immunoglobulin single variable domains specifically targeting PD-L1 that show potency in bioassays, such as competition with PD1 in flow- cytometry or functionality in a tetanus-toxoid autogenic antigen-specific co-culture assay (tetanus- toxoid (TT) assay), with comparable or better IC50 than benchmark(s). The immunoglobulin single variable domains of the present technology specifically bind human, cynomolgus and mouse PD-L1 with maximal 5-fold difference in KD. The present technology provides immunoglobulin single variable domains that do not bind PD-L2 and / or that bind PD-L2 with a KDthat is higher / worse than 10-4M. The present technology provides immunoglobulin single variable domains that do not bind other members of the B7 family and / or that bind other members of the B7 family with a KD that is higher / worse than 10-4M. The immunoglobulin single variable domains of the present technology and polypeptides and constructs comprising said immunoglobulin single variable domains showed a high potency in competition assays with PD-1 and CD80 and a high potency and efficacy in TT assay, and could be efficiently produced (e.g. in microbial hosts such as Pichia, e.g. P. pastoris). In particular, the present technology provides the following embodiments: 3.1 Embodiments Family 7 In some embodiments, the ISVDs specifically binding to PD-L1 are ISVDs that block the interaction of PD-L1 with PD1. The ISVDs specifically binding to PD-L1 bind to the PD1 binding site on PD-L1. In some embodiments the ISVDs specifically binding to PD-L1 interact with a conformational epitope on PD-L1. In some embodiments the ISVDs specifically binding to PD-L1 are ISVDs that interact with following amino acids of the PD-L1 protein: Y56, E58, E60, D62, N63, Q66, H78 and / or R113. Blocking of this epitope on the PD-L1 protein by a monovalent immunoglobulin single variable domain could already very efficiently prevent PD1 binding and subsequent signal transduction Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding to human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), that interacts with one or more amino acids of the PD-L1 protein selected from Y56, E58, E60, D62, N63, Q66, H78 and R113. Embodiment 2. The immunoglobulin single variable domain (ISVD) of embodiment 1, that interacts with following amino acids of the PD-L1 protein: Y56, E58, D62, N63, Q66 and H78. Embodiment 3. The immunoglobulin single variable domain (ISVD) of any of embodiment 1 or 2, that interacts with following amino acids of the PD-L1 protein: Y56, E58, E60, D62, N63, Q66, H78 and R113. Embodiment 4. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 3, in which CDR2 and CDR3 interact with one or more amino acids of the PD-L1 protein selected from Y56, E58, E60, D62, N63, Q66, H78 and R113. Embodiment 5. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 4, in which CDR2 and CDR3 interact with following amino acids of the PD-L1 protein: Y56, E58, D62, N63, Q66 and H78. Embodiment 6. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 5, in which CDR2 and CDR3 interact with following amino acids of the PD-L1 protein: Y56, E58, E60, D62, N63, Q66, H78 and R113. Embodiment 7. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 6, in which S51, S53, and N56 (Kabat numbering) in CDR2 form the interaction site with the amino acids of the PD-L1 protein. Embodiment 8. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 7, in which A98, A99, A100, A100a, I100d, and I100g (Kabat numbering) in CDR3 form the interaction site with the amino acids of the PD-L1 protein. Embodiment 9. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 8, in which S51, S53, and N56 (Kabat numbering) in CDR2 and A98, A99, A100, A100a, I100d, and I100g (Kabat numbering) in CDR3 form the interaction site with the epitope on the PD-L1 protein. Embodiment 10. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 9, in which S51, S53, and N56 (Kabat numbering) in CDR2 and A98, A99, A100, A100a, I100d, and I100g (Kabat numbering) in CDR3 form the interaction site with one or more amino acids of the PD- L1 protein selected from Y56, E58, E60, D62, N63, Q66, H78 and R113. Embodiment 11. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 8, in which S51, S53, and N56 (Kabat numbering) in CDR2 and A98, A99, A100, A100a, I100d, and I100g (Kabat numbering) in CDR3 form the interaction site with following amino acids of the PD-L1 protein: Y56, E58, D62, N63, Q66 and H78. Embodiment 12. The immunoglobulin single variable domain (ISVD) of any of embodiments 1 to 8, in which S51, S53, and N56 (Kabat numbering) in CDR2 and A98, A99, A100, A100a, I100d, and I100g (Kabat numbering) in CDR3 form the interaction site with following amino acids of the PD-L1 protein: Y56, E58, E60, D62, N63, Q66, H78 and R113. Embodiment 13. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (AbM numbering) consists of an amino acid sequence X1X2SX3SX4X5NX6X7(SEQ ID NO: 600), wherein X1, X2, X3, X4, X5, X6, and X7 are any amino acid independently chosen. and - CDR3 (AbM numbering) consists of an amino acid sequence X1X2X3AAAAX4X5IX6X7IX8X9X10 (SEQ ID NO: 603), wherein X1, X2, X3, X4, X5, X6, X7, X8, X9 and X10are any amino acid independently chosen. Embodiment 14. The immunoglobulin single variable domain (ISVD) according to embodiment 13, wherein - CDR2 (AbM numbering) consists of an amino acid sequence X1X2SX3SX4X5NX6X7 (SEQ ID NO: 600), wherein X1 is selected from A, S, T, P and G, X2 is selected from M, L, I, V and C, X3 is selected from A, S, T, P and G, X4 is selected from A, S, T, P and G, X5 is selected from R, H and K, X6 is selected from A, S, T, P and G, and X7 is selected from N and Q; and - CDR3 (AbM numbering) consists of an amino acid sequence X1X2X3AAAAX4X5IX6X7IX8X9X10(SEQ ID NO: 603), wherein X1is selected from A, S, T, P and G, X2is selected from A, S, T, P and G, X3is selected from F, Y, and W, X4is selected from A, S, T, P and G, X5 is selected from A, S, T, P and G, X6 is selected from A, S, T, P and G, X7 is selected from N, Q, I, L, M, V and C, X8 is selected from F, Y, and W, X9 is selected from D and E and X10 is selected from F, Y, and W. Embodiment 15. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 13 or 14; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 13 or 14; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 13 or 14; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NOs: 17 or 109; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; f) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 109. Embodiment 16. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3 is selected from N, T and F; b) amino acid sequences that have at least 80% amino acid identity with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1is selected from A and G, the amino acid residue X2is selected from S and T, and the amino acid residue X3 is selected from N, T and F; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3is selected from N, T and F; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, the amino acid residue X3 is selected from A, P and S, and the amino acid residue X4 is selected from N and I; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1is selected from A and P, the amino acid residue X2is selected from I, L and T, and the amino acid residue X3 is selected from N and I; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, and the amino acid residue X3is selected from N and I. Embodiment 17. The ISVD according to embodiments 13 to 16, in which the amino acid sequences of the CDR2 and CDR3 (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDR2 and CDR3 of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324. Embodiment 18. The ISVD according to any one of embodiments 13 to 17, in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 111, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 104; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 112. Embodiment 19. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 23 or 24; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 23 or 24; c) amino acid sequences that have 6, 5, 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 23 or 24; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NOs: 17 or 109; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; f) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 39. Embodiment 20. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6(SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from S and T, wherein the amino acid residue X3is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6 (SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from S and T, wherein the amino acid residue X3is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6(SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from S and T, wherein the amino acid residue X3is selected from T, N and F, wherein the amino acid residue X4is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2 is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2 is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2is selected from I, L and T, wherein the amino acid residue X3is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N. Embodiment 21. The ISVD according to embodiments 19 or 20, in which the amino acid sequences of the CDR2 and CDR3 (Kabat numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDR2 and CD3 of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324. Embodiment 22. The ISVD according to any one of embodiments 19 to 21, in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 23; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17), or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 122; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 123; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 111. Embodiment 23. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 11 or 98; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 11 or 98; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 11 or 98; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 13 or 14; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 13 or 14; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 13 or 14; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 17 or 109; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 109. Embodiment 24. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence GRX1FSX2NX3MG (SEQ ID NO: 596), wherein the amino acid residue X1 is selected from T, K and G, the amino acid residue X2 is selected from G and R, and the amino acid residue X3 is selected from T and A; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence GRX1FSX2NX3MG (SEQ ID NO: 596), wherein the amino acid residue X1is selected from T, K and G, the amino acid residue X2is selected from G and R, and the amino acid residue X3 is selected from T and A; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence GRX1FSX2NX3MG (SEQ ID NO: 596), wherein the amino acid residue X1 is selected from T, K and G, the amino acid residue X2 is selected from G and R, and the amino acid residue X3 is selected from T and A; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3 is selected from N, T and F; e) amino acid sequences that have at least 80% amino acid identity with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1is selected from A and G, the amino acid residue X2is selected from S and T, and the amino acid residue X3is selected from N, T and F; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3 is selected from N, T and F; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, the amino acid residue X3 is selected from A, P and S, and the amino acid residue X4 is selected from N and I; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, and the amino acid residue X3 is selected from N and I; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1is selected from A and P, the amino acid residue X2is selected from I, L and T, and the amino acid residue X3is selected from N and I. Embodiment 25. The ISVD according to embodiments 23 or 24, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324. Embodiment 26. The ISVD according to any one of embodiments 23 to 25, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 11; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1 consists of the amino acid sequence of SEQ ID NO: 11; ^ CDR2 consists of one of the amino acid sequences of SEQ ID NOs: 13 and 14; and ^ CDR3 consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 98; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 98; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 99; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 99; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 11; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 100; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 100; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 111, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 101; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 104; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 112. Embodiment 27. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 22; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 22; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 22; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 23 or 24; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 23 or 24; f) amino acid sequences that have 6, 5, 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 23 or 24; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 17 or 109; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 109. Embodiment 28. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of X1NX2MG (SEQ ID NO: 604); wherein the amino acid residue X1is selected from G and R and wherein the amino acid residue X2is selected from T and A; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1NX2MG (SEQ ID NO: 604); wherein the amino acid residue X1is selected from G and R and wherein the amino acid residue X2 is selected from T and A; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence X1NX2MG (SEQ ID NO: 604); wherein the amino acid residue X1 is selected from G and R and wherein the amino acid residue X2 is selected from T and A; and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6(SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6(SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6(SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2is selected from I, L and T, wherein the amino acid residue X3is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N. Embodiment 29. The ISVD according to embodiments 27 or 28, in which the amino acid sequences of the CDRs (Kabat numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324. Embodiment 30. The ISVD according to any one of embodiments 27 to 29, in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2 (Kabat numbering) consists of one of the amino acid sequences of SEQ ID NO: 23 and 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 23; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:22; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 118; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 122; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 118; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 123; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 111, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 118; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 123; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 111. Embodiment 31. The ISVD according to any of one embodiments 1 to 30, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 7, and SEQ ID NOs: 311-324, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 32. The ISVD according to any one of embodiments 1 to 31, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 33. The ISVD according to any one of embodiments 1 to 32, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 34. The ISVD according to any one of embodiments 1 to 33, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NO: 5 and SEQ ID NOs: 311-321 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with at least one of the amino acid sequences of SEQ ID NO: 4, 5 and SEQ ID NOs: 311-321. Embodiment 35. The ISVD according to any one of embodiments 1 to 34, in which the amino acid sequence is chosen from the group consisting of SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 7 and SEQ ID NOs: 311-339. Embodiment 36. The ISVD according to any one of embodiments 1 to 35, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 5.10-9to 10-11moles / litre or less, and preferably 10-9to 5.10-11moles / litre or 5.10-10to 10-10moles / litre or more preferably 10-10to 10-11moles / litre , as determined by Surface Plasmon Resonance. Embodiment 37. The ISVD according to any one of embodiments 1 to 36, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 38. The ISVD according to any one of embodiments 1 to 37, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-3s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-3s-1and 5.10-6s-1, more preferably between 5.10-4s-1and 5.10-6s-1, such as between 5.10-4s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 39. The ISVD according to any one of embodiments 1 to 38, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 40. The ISVD according to any one of embodiments 1 to 39, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 41. The ISVD according to any one of embodiments 1 to 40, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-8M or lower, more preferably of 10-9M or lower, or even of 5.10-10M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 10-8M, such as between 10-10M and 10-8M, between 10-10M and 10-9M or between 10-11M and 10-9M. Embodiment 42. The ISVD according to any one of embodiments 1 to 41, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 43. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 42, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 44. The polypeptide or construct according to embodiment 43, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 526, 529, 533, 541, 542, 546, 547, 548, 551, 552, 554, 560, 563, 572, 582, 583, 590, and 593. Embodiment 45. The polypeptide or construct according to any one of embodiments 43 to 44, in which said one or more linkers are one or more amino acid sequences. Embodiment 46. The polypeptide or construct according to any one of embodiments 43 to 45, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 47. The polypeptide or construct according to any one of embodiments 43 to 46, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 48. The polypeptide or construct according to any one of embodiments 43 to 47, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 49. The polypeptide or construct according to any one of embodiments 43 to 48, which is a multivalent construct. Embodiment 50. The polypeptide or construct according to any one of embodiments 43 to 49, which is a multispecific construct. Embodiment 51. The polypeptide or construct according to any one of embodiments 46 to 50, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 52. The polypeptide or construct according to embodiment 51, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 53. The polypeptide or construct according to embodiment 52, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 54. The polypeptide or construct according to embodiment 52, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 55. The polypeptide or construct according to embodiment 54, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of ISVDs, VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 56. The polypeptide or construct according to embodiment 55, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 57. The polypeptide or construct according to embodiment 56, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 58. The polypeptide or construct according to embodiment 57, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 59. The polypeptide or construct according to embodiment 54, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 60. The polypeptide or construct according to anyone of embodiments 43 to 59, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 61. The polypeptide or construct according to any of embodiments 43 to 60, further comprising a C-terminal extension. Embodiment 62. The polypeptide or construct according to embodiment 61, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 63. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 42, or a polypeptide according to any one of embodiments 43 to 60. Embodiment 64. The nucleic acid according to embodiment 63, that is in the form of a genetic construct. Embodiment 65. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 42, or a polypeptide according to any one of embodiments 43 to 62; and / or that comprises the nucleic acid according to any one of embodiments 63 or 64. Embodiment 66. A method for producing an ISVD according to any one of embodiments 1 to 42, or a polypeptide according to any one of embodiments 43 to 62, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 63 or 64; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 42, or the polypeptide according to any one of embodiments 43 to 62. Embodiment 67. A method for producing an ISVD according to any one of embodiments 1 to 42, or a polypeptide according to any one of embodiments 43 to 62, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 65 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 42, or at least one polypeptide according to any one of embodiments 43 to 62; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 42, or polypeptide according to any one of embodiments 43 to 62. Embodiment 68. A composition comprising at least one ISVD according to any one of embodiments 1 to 42, at least one polypeptide or construct according to any one of embodiments 43 to 62, or at least one nucleic acid according to any one of embodiments 63 or 64. Embodiment 69. The composition according to embodiment 68, which is a pharmaceutical composition. Embodiment 70. The composition according to any one of embodiments 68 or 69, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 71. The ISVD according to any one of embodiments 1 to 42, the polypeptide or construct according to any one of embodiments 43 to 62, or the composition according to any one of embodiments 68 to 70, for use as a medicament. Embodiment 72. The ISVD according to any one of embodiments 1 to 42, the polypeptide or construct according to any one of embodiments 43 to 62, or the composition according to any one of embodiments 68 to 70, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 73. The ISVD according to any one of embodiments 1 to 42, the polypeptide or construct according to any one of embodiments 43 to 62, or the composition according to any one of embodiments 68 to 70, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 74. The ISVD according to any one of embodiments 1 to 42, the polypeptide or construct according to any one of embodiments 43 to 62, or the composition according to any one of embodiments 68 to 70, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 75. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 42, a polypeptide or construct according to any one of embodiments 43 to 62, or a composition according to any one of embodiments 68 to 70. Embodiment 76. The method according to embodiment 75, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 42, a polypeptide or construct according to any one of embodiments 43 to 62, or composition according to any one of embodiments 68 to 70. Embodiment 77. The method according to any one of embodiments 75 or 76, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 42, a polypeptide or construct according to any one of embodiments 43 to 62, or a composition according to any one of embodiments 68 to 70. 3.2 Embodiments Family 42 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 132; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 132; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 132; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 134 or 136; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 134 or 136; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 134 or 136; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 143 or 145; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 143 or 145; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 143 or 145. Embodiment 2. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence GGTFRHYVMG (SEQ ID NO: 132); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence GGTFRHYVMG (SEQ ID NO: 132); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence GGTFRHYVMG (SEQ ID NO: 132); and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1X2SWSGSGX3Y (SEQ ID NO: 607), wherein the amino acid residue X1 is selected from A or G, the amino acid residue X2 is selected from I, S and V, and the amino acid residue X3 is selected from T and Q; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1X2SWSGSGX3Y (SEQ ID NO: 607), wherein the amino acid residue X1is selected from A or G, the amino acid residue X2is selected from I, S and V, and the amino acid residue X3 is selected from T and Q; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1X2SWSGSGX3Y (SEQ ID NO: 607), wherein the amino acid residue X1 is selected from A or G, the amino acid residue X2 is selected from I, S and V, and the amino acid residue X3is selected from T and Q; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608), wherein the amino acid residue X1 is selected from A, V, R, K and E, the amino acid residue X2 is selected from Q and G, the amino acid residue X3 is selected from V, K, Q, H and E, and the amino acid residue X4 is selected from L and R; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608), wherein the amino acid residue X1 is selected from A, V, R, K and E, the amino acid residue X2 is selected from Q and G, the amino acid residue X3 is selected from V, K, Q, H and E, and the amino acid residue X4 is selected from L and R;; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608), wherein the amino acid residue X1is selected from A, V, R, K and E, the amino acid residue X2is selected from Q and G, the amino acid residue X3 is selected from V, K, Q, H and E, and the amino acid residue X4 is selected from L and R. Embodiment 3. The ISVD according to embodiments 1 or 2, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 340-379. Embodiment 4. The ISVD according to any one of embodiments 1 to 3, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 134; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 143, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 148, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 149, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 150, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 149, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 149, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 150, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 151, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 137; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 152. Embodiment 5. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 159; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 159; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 159; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 160 or 162; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 160 or 162; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 160 or 162; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 143 or 145; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 143 or 145; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 143 or 145. Embodiment 6. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of HYVMG (SEQ ID NO: 159); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of HYVMG (SEQ ID NO: 159); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of HYVMG (SEQ ID NO: 159); and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1X2SWSGSGX3YYADSVKG (SEQ ID NO: 610); wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from I, S and V, and wherein the amino acid residue X3is selected from T and Q; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1X2SWSGSGX3YYADSVKG (SEQ ID NO: 610); wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from I, S and V, and wherein the amino acid residue X3 is selected from T and Q; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1X2SWSGSGX3YYADSVKG (SEQ ID NO: 610); wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from I, S and V, and wherein the amino acid residue X3 is selected from T and Q; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608); wherein the amino acid residue X1is selected from A, V, R, K or E, wherein the amino acid residue X2 is selected from Q or G, wherein the amino acid residue X3 is selected from V, K, Q, H or E, and wherein the amino acid residue X4is selected from R or L; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608); wherein the amino acid residue X1is selected from A, V, R, K or E, wherein the amino acid residue X2 is selected from Q or G, wherein the amino acid residue X3 is selected from V, K, Q, H or E, and wherein the amino acid residue X4 is selected from R or L; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608); wherein the amino acid residue X1is selected from A, V, R, K or E, wherein the amino acid residue X2is selected from Q or G, wherein the amino acid residue X3is selected from V, K, Q, H or E, and wherein the amino acid residue X4 is selected from R or L. Embodiment 7. The ISVD according to embodiments 5 or 6, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 340-379. Embodiment 8. The ISVD according to any one of embodiments 5 to 7, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 160; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 143, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 148, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 149, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 150. or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 148, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 166; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 149. or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 150, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 151, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 163; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 152. Embodiment 9. The ISVD according to any of one embodiments 1 to 8, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 340-379, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 341-378 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with at least one of the amino acid sequences of SEQ ID NOs: 341-378. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 340-419. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 5.10-9to 10-11moles / litre or less, and preferably 10-9to 5.10-11moles / litre or 5.10-10to 10-10moles / litre or more preferably 10-10to 10-11moles / litre, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 104M-1s-1to about 107M-1s-1, preferably between 105M-1s-1and 107M-1s-1, more preferably between 5.105M-1s-1and 107M-1s-1, such as between 105M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-3s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-3s-1and 5.10-6s-1, more preferably between 5.10-4s-1and 5.10-6s-1, such as between 5.10-4s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 19. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-8M or lower, more preferably of 10-9M or lower, or even of 5.10-10M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 10-8M, such as between 10-10M and 10-8M, between 10-10M and 10-9M or between 10-11M and 10-9M. Embodiment 20. The ISVD according to any one of embodiments 1 to 19, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 21. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 20, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 22. The polypeptide or construct according to embodiment 21, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 559, 561, 568, 575, 578, 580, 585, 587, 589, 591, and 595. Embodiment 23. The polypeptide or construct according to any one of embodiments 21 to 22, in which said one or more linkers are one or more amino acid sequences. Embodiment 24. The polypeptide or construct according to any one of embodiments 21 to 23, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 25. The polypeptide or construct according to any one of embodiments 21 to 24, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 26. The polypeptide or construct according to any one of embodiments 21 to 25, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 27. The polypeptide or construct according to any one of embodiments 21 to 26, which is a multivalent construct. Embodiment 28. The polypeptide or construct according to any one of embodiments 21 to 27, which is a multispecific construct. Embodiment 29. The polypeptide or construct according to any one of embodiments 24 to 28, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 30. The polypeptide or construct according to embodiment 29, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 32. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 33. The polypeptide or construct according to embodiment 32, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 34. The polypeptide or construct according to embodiment 33, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 35. The polypeptide or construct according to embodiment 34, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 36. The polypeptide or construct according to embodiment 35, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 37. The polypeptide or construct according to embodiment 34, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 38. The polypeptide or construct according to anyone of embodiments 21 to 37, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 39. The polypeptide or construct according to any of embodiments 21 to 38, further comprising a C-terminal extension. Embodiment 40. The polypeptide or construct according to embodiment 39, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 41. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40. Embodiment 42. The nucleic acid according to embodiment 41, that is in the form of a genetic construct. Embodiment 43. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40; and / or that comprises the nucleic acid according to any one of embodiments 41 or 42. Embodiment 44. A method for producing an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 41 or 42; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 20, or the polypeptide according to any one of embodiments 21 to 40. Embodiment 45. A method for producing an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 20, or at least one polypeptide according to any one of embodiments 21 to 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 20, or polypeptide according to any one of embodiments 21 to 40. Embodiment 46. A composition comprising at least one ISVD according to any one of embodiments 1 to 20, at least one polypeptide or construct according to any one of embodiments 21 to 40, or at least one nucleic acid according to any one of embodiments 41 or 42. Embodiment 47. The composition according to embodiment 46, which is a pharmaceutical composition. Embodiment 48. The composition according to any one of embodiments 46 or 47, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 49. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use as a medicament. Embodiment 50. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 51. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 52. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 53. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or a composition according to any one of embodiments 46 to 47. Embodiment 54. The method according to embodiment 53, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or composition according to any one of embodiments 46 to 47. Embodiment 55. The method according to any one of embodiments 53 or 54, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or a composition according to any one of embodiments 46 to 47. 3.3 Embodiments Family 18 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 169; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 169; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 169; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 171 or 175; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 171 or 175; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 171 or 175; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 192 or 193; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 192 or 193; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 192 or 193. Embodiment 2. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence GSIFTSAVME (SEQ ID NO: 169); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence GSIFTSAVME (SEQ ID NO: 169); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence GSIFTSAVME (SEQ ID NO: 169); and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of RIGSGGRX1A (SEQ ID NO: 611), wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q, A, S,P, and G; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RIGSGGRX1A (SEQ ID NO: 611), wherein the amino acid residue X1 is selected from I, R, H, E, K, T, Q, A, S,P, and G; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RIGSGGRX1A (SEQ ID NO: 611), wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q, A, S,P, and G; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of VX1GGYIY (SEQ ID NO: 612), wherein the amino acid residue X1 is selected from M and I; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612), wherein the amino acid residue X1 is selected from M and I; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612), wherein the amino acid residue X1 is selected from M and I. Embodiment 3. The ISVD according to embodiments 1 or 2, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 420-450. Embodiment 4. The ISVD according to any one of embodiments 1 to 3, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 171; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 192, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 172; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 173; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 174; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 175; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 176; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 177; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 178; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 171; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 179; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 180; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 181; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193. Embodiment 5. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 197; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 197; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 197; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 198 or 202; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 198 or 202; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 198 or 202; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 192 or 193; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 192 or 193; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 192 or 193. Embodiment 6. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SAVME (SEQ ID NO: 197); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SAVME (SEQ ID NO: 197); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SAVME (SEQ ID NO: 197); and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of RIGSGGRX1AYX2DSVKG (SEQ ID NO: 613); wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q; A, S, P and G, and wherein the amino acid residue X2is selected from P and V; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RIGSGGRX1AYX2DSVKG (SEQ ID NO: 613); wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q; A, S, P and G, and wherein the amino acid residue X2 is selected from P and V; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RIGSGGRX1AYX2DSVKG (SEQ ID NO: 613); wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q; A, S, P and G, and wherein the amino acid residue X2is selected from P and V; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of VX1GGYIY (SEQ ID NO: 612); wherein the amino acid residue X1is selected from M and I; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612); wherein the amino acid residue X1is selected from M and I; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612); wherein the amino acid residue X1is selected from M and I. Embodiment 7. The ISVD according to embodiments 5 or 6, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 420-450. Embodiment 8. The ISVD according to any one of embodiments 5 to 7, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 198; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 192, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 199; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 200; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 201; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 202; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 203; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 204; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 205; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193. or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 198; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 206; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 207; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 208; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 209; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193. Embodiment 9. The ISVD according to any of one embodiments 1 to 8, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 420-450, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 421-449 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with at least one of the amino acid sequences of SEQ ID NOs: 421-449. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 420-481. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 5.10-9to 10-11moles / litre or less, and preferably 10-9to 5.10-11moles / litre or 10-9to 10-10moles / litre or more preferably 10-9to 10-11moles / litre, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 104M-1s-1to about 107M-1s-1, preferably between 5.104M-1s-1and 107M-1s-1, more preferably between 105M-1s-1and 107M-1s-1, such as between 5.105M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-3s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-3s-1and 5.10-6s-1, more preferably between 10-4s-1and 10-6s-1, such as between 10-4s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 19. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g., as described in the Examples), monitored via IFN-gamma production, of 10-8M or lower, more preferably of 10-9M or lower, or even of 5.10-10M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 10-8M, such as between 10-10M and 10-8M, between 10-10M and 5.10-9M or between 10-11M and 5.10-9M. Embodiment 20. The ISVD according to any one of embodiments 1 to 19, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 21. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 20, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 22. The polypeptide or construct according to embodiment 21, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 535, 536, 538, 570, 576, 581, 586, 588, 592, and 594. Embodiment 23. The polypeptide or construct according to any one of embodiments 21 to 22, in which said one or more linkers are one or more amino acid sequences. Embodiment 24. The polypeptide or construct according to any one of embodiments 21 to 23, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 25. The polypeptide or construct according to any one of embodiments 21 to 24, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 26. The polypeptide or construct according to any one of embodiments 21 to 25, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 27. The polypeptide or construct according to any one of embodiments 21 to 26, which is a multivalent construct. Embodiment 28. The polypeptide or construct according to any one of embodiments 21 to 27, which is a multispecific construct. Embodiment 29. The polypeptide or construct according to any one of embodiments 24 to 28, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 30. The polypeptide or construct according to embodiment 29, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 32. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 33. The polypeptide or construct according to embodiment 32, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 34. The polypeptide or construct according to embodiment 33, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 35. The polypeptide or construct according to embodiment 34, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 36. The polypeptide or construct according to embodiment 35, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 37. The polypeptide or construct according to embodiment 34, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 38. The polypeptide or construct according to anyone of embodiments 21 to 37, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 39. The polypeptide or construct according to any of embodiments 21 to 38, further comprising a C-terminal extension. Embodiment 40. The polypeptide or construct according to embodiment 39, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 41. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40. Embodiment 42. The nucleic acid according to embodiment 41, that is in the form of a genetic construct. Embodiment 43. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40; and / or that comprises the nucleic acid according to any one of embodiments 41 or 42. Embodiment 44. A method for producing an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 41 or 42; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 20, or the polypeptide according to any one of embodiments 21 to 40. Embodiment 45. A method for producing an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 20, or at least one polypeptide according to any one of embodiments 21 to 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 20, or polypeptide according to any one of embodiments 21 to 40. Embodiment 46. A composition comprising at least one ISVD according to any one of embodiments 1 to 20, at least one polypeptide or construct according to any one of embodiments 21 to 40, or at least one nucleic acid according to any one of embodiments 41 or 42. Embodiment 47. The composition according to embodiment 46, which is a pharmaceutical composition. Embodiment 48. The composition according to any one of embodiments 46 or 47, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 49. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use as a medicament. Embodiment 50. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 51. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 52. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 53. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or a composition according to any one of embodiments 46 to 47. Embodiment 54. The method according to embodiment 53, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or composition according to any one of embodiments 46 to 47. Embodiment 55. The method according to any one of embodiments 53 or 54, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or a composition according to any one of embodiments 46 to 47. 3.4 Embodiments Family 16 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 218; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 218; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 218; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 222; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 222; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 222; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 234; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 234; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 234. Embodiment 2. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence ERTFX1SYSMG (SEQ ID NO: 617), wherein the amino acid residue X1 is selected from S and G; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence ERTFX1SYSMG (SEQ ID NO: 617), wherein the amino acid residue X1 is selected from S and G; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence ERTFX1SYSMG (SEQ ID NO: 617), wherein the amino acid residue X1is selected from S and G; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1IX2YRGAGPY (SEQ ID NO: 614), wherein the amino acid residue X1 is selected from V and P, and wherein the amino acid residue X2 is selected from G and F; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1IX2YRGAGPY (SEQ ID NO: 614), wherein the amino acid residue X1 is selected from V and P, and wherein the amino acid residue X2 is selected from G and F; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1IX2YRGAGPY (SEQ ID NO: 614), wherein the amino acid residue X1 is selected from V and P, and wherein the amino acid residue X2 is selected from G and F; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615), wherein the amino acid residue X1is selected from A and T; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615), wherein the amino acid residue X1 is selected from A and T; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615), wherein the amino acid residue X1 is selected from A and T. Embodiment 3. The ISVD according to embodiments 1 or 2, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 482-497. Embodiment 4. The ISVD according to any one of embodiments 1 to 3, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 218; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 222; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 218; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 223; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 218; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 224; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 219; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 222; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 235. Embodiment 5. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 241; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 241; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 241; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 242; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 242; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 242; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 234; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 234; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 234. Embodiment 6. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SYSMG (SEQ ID NO: 241); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SYSMG (SEQ ID NO: 241); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SYSMG (SEQ ID NO: 241); and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1IX2YRGAGPYYEDSVKG (SEQ ID NO: 616); wherein the amino acid residue X1is selected V and P, and wherein the amino acid residue X2 is selected from G and F; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1IX2YRGAGPYYEDSVKG (SEQ ID NO: 616); wherein the amino acid residue X1is selected V and P, and wherein the amino acid residue X2 is selected from G and F; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1IX2YRGAGPYYEDSVKG (SEQ ID NO: 616); wherein the amino acid residue X1is selected V and P, and wherein the amino acid residue X2 is selected from G and F; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615); wherein the amino acid residue X1is selected from A and T; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615); wherein the amino acid residue X1is selected from A and T; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615); wherein the amino acid residue X1is selected from A and T. Embodiment 7. The ISVD according to embodiments 5 or 6, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 482-497. Embodiment 8. The ISVD according to any one of embodiments 5 to 7, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 242; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 243; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 244; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 242; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 235. Embodiment 9. The ISVD according to any of one embodiments 1 to 8, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 482-497, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 483-496 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with at least one of the amino acid sequences of SEQ ID NOs: 483-496. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 482-513. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 5.10-9to 10-11moles / litre or less, and preferably 10-9to 5.10-11moles / litre or 5.10-10to 10-10moles / litre or more preferably 10-9to 10-10moles / litre, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-3s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-3s-1and 5.10-5s-1, more preferably between 5.10-4s-1and 5.10-5s-1, such as between 10-4s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 19. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-8M or lower, more preferably of 10-9M or lower, or even of 5.10-10M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 10-8M, such as between 10-10M and 10-8M, between 10-10M and 10-9M or between 10-11M and 10-9M. Embodiment 20. The ISVD according to any one of embodiments 1 to 19, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 21. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 20, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 22. The polypeptide or construct according to embodiment 21, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 531, 549, 567, 574, 579, 584. Embodiment 23. The polypeptide or construct according to any one of embodiments 21 to 22, in which said one or more linkers are one or more amino acid sequences. Embodiment 24. The polypeptide or construct according to any one of embodiments 21 to 23, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 25. The polypeptide or construct according to any one of embodiments 21 to 24, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 26. The polypeptide or construct according to any one of embodiments 21 to 25, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 27. The polypeptide or construct according to any one of embodiments 21 to 26, which is a multivalent construct. Embodiment 28. The polypeptide or construct according to any one of embodiments 21 to 27, which is a multispecific construct. Embodiment 29. The polypeptide or construct according to any one of embodiments 24 to 28, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 30. The polypeptide or construct according to embodiment 29, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 32. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 33. The polypeptide or construct according to embodiment 32, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 34. The polypeptide or construct according to embodiment 33, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 35. The polypeptide or construct according to embodiment 34, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 36. The polypeptide or construct according to embodiment 35, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 37. The polypeptide or construct according to embodiment 34, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 38. The polypeptide or construct according to anyone of embodiments 21 to 37, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 39. The polypeptide or construct according to any of embodiments 21 to 38, further comprising a C-terminal extension. Embodiment 40. The polypeptide or construct according to embodiment 39, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 41. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40. Embodiment 42. The nucleic acid according to embodiment 41, that is in the form of a genetic construct. Embodiment 43. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40; and / or that comprises the nucleic acid according to any one of embodiments 41 or 42. Embodiment 44. A method for producing an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 41 or 42; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 20, or the polypeptide according to any one of embodiments 21 to 40. Embodiment 45. A method for producing an ISVD according to any one of embodiments 1 to 20, or a polypeptide according to any one of embodiments 21 to 40, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 20, or at least one polypeptide according to any one of embodiments 21 to 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 20, or polypeptide according to any one of embodiments 21 to 40. Embodiment 46. A composition comprising at least one ISVD according to any one of embodiments 1 to 20, at least one polypeptide or construct according to any one of embodiments 21 to 40, or at least one nucleic acid according to any one of embodiments 41 or 42. Embodiment 47. The composition according to embodiment 46, which is a pharmaceutical composition. Embodiment 48. The composition according to any one of embodiments 46 or 47, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 49. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use as a medicament. Embodiment 50. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 51. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 52. The ISVD according to any one of embodiments 1 to 20, the polypeptide or construct according to any one of embodiments 21 to 40, or the composition according to any one of embodiments 46 to 47, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 53. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or a composition according to any one of embodiments 46 to 47. Embodiment 54. The method according to embodiment 53, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or composition according to any one of embodiments 46 to 47. Embodiment 55. The method according to any one of embodiments 53 or 54, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 20, a polypeptide or construct according to any one of embodiments 21 to 40, or a composition according to any one of embodiments 46 to 47. 3.5 Embodiments Family 17 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 256; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 256; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 256; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 267; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 267; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 267; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 279; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 279; i) amino acid sequences that have 43, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 279. Embodiment 2. The ISVD according to embodiment 1, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 514. Embodiment 3. The ISVD according to any one of embodiments 1 to 2, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 256; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 267; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 279. Embodiment 4. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 293; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 293; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 293; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 299; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 299; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 299; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 279; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 279; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 279. Embodiment 5. The ISVD according to embodiment 4, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 514. Embodiment 6. The ISVD according to any one of embodiments 4 to 5, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 293; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 299; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 279. Embodiment 7. The ISVD according to any of one embodiments 1 to 6, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 514, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 8. The ISVD according to any one of embodiments 1 to 7, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 9. The ISVD according to any one of embodiments 1 to 8, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 514 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with the amino acid sequence of SEQ ID NOs: 514. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 514 and 520. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 10-8to 10-11moles / litre or less, and preferably 5.10-9to 5.10-11moles / litre or 5.10-9to 10-10moles / litre or more preferably 10-8to 10-10moles / litre, as determined by Surface Plasmon Resonance. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-2s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 5.10-3s-1and 5.10-6s-1, more preferably between 5.10-3s-1and 10-5s-1, such as between 10-3s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-7M or lower, more preferably of 5.10-8M or lower, or even of 10-8M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 10-7M, such as between 10-10M and 10-7M, between 10-10M and 5.10-8. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 19. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 18, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 20. The polypeptide or construct according to embodiment 19, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 543, 545, and 548. Embodiment 21. The polypeptide or construct according to any one of embodiments 19 to 20, in which said one or more linkers are one or more amino acid sequences. Embodiment 22. The polypeptide or construct according to any one of embodiments 19 to 21, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 23. The polypeptide or construct according to any one of embodiments 19 to 22, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 24. The polypeptide or construct according to any one of embodiments 19 to 23, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 25. The polypeptide or construct according to any one of embodiments 19 to 24, which is a multivalent construct. Embodiment 26. The polypeptide or construct according to any one of embodiments 19 to 25, which is a multispecific construct. Embodiment 27. The polypeptide or construct according to any one of embodiments 22 to 26, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 28. The polypeptide or construct according to embodiment 27, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 29. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 30. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 32. The polypeptide or construct according to embodiment 31, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 33. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 34. The polypeptide or construct according to embodiment 33, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 35. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 36. The polypeptide or construct according to anyone of embodiments 19 to 35, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 37. The polypeptide or construct according to any of embodiments 19 to 36, further comprising a C-terminal extension. Embodiment 38. The polypeptide or construct according to embodiment 37, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 39. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38. Embodiment 40. The nucleic acid according to embodiment 39, that is in the form of a genetic construct. Embodiment 41. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 39; and / or that comprises the nucleic acid according to any one of embodiments 39 or 40. Embodiment 42. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 39 or 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or the polypeptide according to any one of embodiments 19 to 38. Embodiment 43. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 18, or at least one polypeptide according to any one of embodiments 19 to 38; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or polypeptide according to any one of embodiments 19 to 38. Embodiment 44. A composition comprising at least one ISVD according to any one of embodiments 1 to 18, at least one polypeptide or construct according to any one of embodiments 19 to 38, or at least one nucleic acid according to any one of embodiments 39 or 40. Embodiment 45. The composition according to embodiment 44, which is a pharmaceutical composition. Embodiment 46. The composition according to any one of embodiments 44 or 45, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 47. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use as a medicament. Embodiment 48. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 49. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 50. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 51. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. Embodiment 52. The method according to embodiment 51, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or composition according to any one of embodiments 44 to 45. Embodiment 53. The method according to any one of embodiments 51 or 52, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. 3.6 Embodiments Family 6 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 257; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 257; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 257; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 268; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 268; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 268; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 280; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 280; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 280. Embodiment 2. The ISVD according to embodiment 1, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 515. Embodiment 3. The ISVD according to any one of embodiments 1 to 2, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 257; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 268; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 280. Embodiment 4. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 294; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 294; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 294; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 300; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 300; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 300; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 280; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 280; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 280. Embodiment 5. The ISVD according to embodiment 4, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 515. Embodiment 6. The ISVD according to any one of embodiments 4 to 5, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 294; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 300; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 280. Embodiment 7. The ISVD according to any of one embodiments 1 to 6, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 515, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 8. The ISVD according to any one of embodiments 1 to 7, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 9. The ISVD according to any one of embodiments 1 to 8, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 514 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with the amino acid sequence of SEQ ID NOs: 514. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 515 and 521. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 10-8to 10-11moles / litre or less, and preferably 10-8to 10-10moles / litre or 5.10-9to 10-11moles / litre or more preferably 5.10-9to 10-10moles / litre, as determined by Surface Plasmon Resonance. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a koffrate between 10-2s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-2s-1and 5.10-6s-1, more preferably between 5.10-3s-1and 5.10-6s-1, such as between 5.10-3s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-8M or lower, more preferably of 10-9M or lower, or even of 5.10-10M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 10-8M, such as between 10-10M and 10-8M, between 10-10M and 10-9M or between 10-11M and 10-9M. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 19. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 18, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 20. The polypeptide or construct according to embodiment 19, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 530 and 539. Embodiment 21. The polypeptide or construct according to any one of embodiments 19 to 20, in which said one or more linkers are one or more amino acid sequences. Embodiment 22. The polypeptide or construct according to any one of embodiments 19 to 21, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 23. The polypeptide or construct according to any one of embodiments 19 to 22, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 24. The polypeptide or construct according to any one of embodiments 19 to 23, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 25. The polypeptide or construct according to any one of embodiments 19 to 24, which is a multivalent construct. Embodiment 26. The polypeptide or construct according to any one of embodiments 19 to 25, which is a multispecific construct. Embodiment 27. The polypeptide or construct according to any one of embodiments 22 to 26, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 28. The polypeptide or construct according to embodiment 27, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 29. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 30. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 32. The polypeptide or construct according to embodiment 31, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 33. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 34. The polypeptide or construct according to embodiment 33, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 35. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 36. The polypeptide or construct according to anyone of embodiments 19 to 35, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 37. The polypeptide or construct according to any of embodiments 19 to 36, further comprising a C-terminal extension. Embodiment 38. The polypeptide or construct according to embodiment 37, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 39. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38. Embodiment 40. The nucleic acid according to embodiment 39, that is in the form of a genetic construct. Embodiment 41. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 39; and / or that comprises the nucleic acid according to any one of embodiments 39 or 40. Embodiment 42. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 39 or 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or the polypeptide according to any one of embodiments 19 to 38. Embodiment 43. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 18, or at least one polypeptide according to any one of embodiments 19 to 38; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or polypeptide according to any one of embodiments 19 to 38. Embodiment 44. A composition comprising at least one ISVD according to any one of embodiments 1 to 18, at least one polypeptide or construct according to any one of embodiments 19 to 38, or at least one nucleic acid according to any one of embodiments 39 or 40. Embodiment 45. The composition according to embodiment 44, which is a pharmaceutical composition. Embodiment 46. The composition according to any one of embodiments 44 or 45, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 47. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use as a medicament. Embodiment 48. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 49. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 50. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 51. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. Embodiment 52. The method according to embodiment 51, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or composition according to any one of embodiments 44 to 45. Embodiment 53. The method according to any one of embodiments 51 or 52, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. 3.7 Embodiments Family 44 Embodiments 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 258; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 258; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 258; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 269; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 269; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 269; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 281; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 281; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 281. Embodiments 2. The ISVD according to embodiment 1, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 516. Embodiments 3. The ISVD according to any one of embodiments 1 to 2, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 258; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 269; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 281. Embodiments 4. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 295; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 295; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 295; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 301; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 301; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 301; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 281; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 281; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 281. Embodiments 5. The ISVD according to embodiment 4, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 516. Embodiments 6. The ISVD according to any one of embodiments 4 to 5, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 295; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 301; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 281. Embodiments 7. The ISVD according to any of one embodiments 1 to 6, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 516, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiments 8. The ISVD according to any one of embodiments 1 to 7, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiments 9. The ISVD according to any one of embodiments 1 to 8, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiments 10. The ISVD according to any one of embodiments 1 to 9, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 516 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with the amino acid sequence of SEQ ID NOs: 516. Embodiments 11. The ISVD according to any one of embodiments 1 to 10, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 516 and 522. Embodiments 12. The ISVD according to any one of embodiments 1 to 11, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 10-8to 10-11moles / litre or less, and preferably 10-8to 10-10moles / litre or 5.10-9to 10-11moles / litre or more preferably 5.10-9to 10-10moles / litre, as determined by Surface Plasmon Resonance. Embodiments 13. The ISVD according to any one of embodiments 1 to 12, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiments 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-2s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-2s-1and 5.10-6s-1, more preferably between 5.10-3s-1and 5.10-6s-1, such as between 5.10-3s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiments 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiments 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiments 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-7M or lower, more preferably of 5.10-8M or lower, or even of 10-8M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 5.10-8M, such as between 10-10M and 5.10-8M, between 10-10M and 10-8M or between 10-11M and 10-8M. Embodiments 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiments 19. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 18, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiments 20. The polypeptide or construct according to embodiment 19, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 556 and 562. Embodiments 21. The polypeptide or construct according to any one of embodiments 19 to 20, in which said one or more linkers are one or more amino acid sequences. Embodiments 22. The polypeptide or construct according to any one of embodiments 19 to 21, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiments 23. The polypeptide or construct according to any one of embodiments 19 to 22, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiments 24. The polypeptide or construct according to any one of embodiments 19 to 23, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiments 25. The polypeptide or construct according to any one of embodiments 19 to 24, which is a multivalent construct. Embodiments 26. The polypeptide or construct according to any one of embodiments 19 to 25, which is a multispecific construct. Embodiments 27. The polypeptide or construct according to any one of embodiments 22 to 26, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiments 28. The polypeptide or construct according to embodiment 27, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiments 29. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiments 30. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiments 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiments 32. The polypeptide or construct according to embodiment 31, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiments 33. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiments 34. The polypeptide or construct according to embodiment 33, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiments 35. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiments 36. The polypeptide or construct according to anyone of embodiments 19 to 35, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiments 37. The polypeptide or construct according to any of embodiments 19 to 36, further comprising a C-terminal extension. Embodiments 38. The polypeptide or construct according to embodiment 37, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiments 39. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38. Embodiments 40. The nucleic acid according to embodiment 39, that is in the form of a genetic construct. Embodiments 41. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 39; and / or that comprises the nucleic acid according to any one of embodiments 39 or 40. Embodiments 42. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 39 or 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or the polypeptide according to any one of embodiments 19 to 38. Embodiments 43. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 18, or at least one polypeptide according to any one of embodiments 19 to 38; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or polypeptide according to any one of embodiments 19 to 38. Embodiments 44. A composition comprising at least one ISVD according to any one of embodiments 1 to 18, at least one polypeptide or construct according to any one of embodiments 19 to 38, or at least one nucleic acid according to any one of embodiments 39 or 40. Embodiments 45. The composition according to embodiment 44, which is a pharmaceutical composition. Embodiments 46. The composition according to any one of embodiments 44 or 45, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiments 47. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use as a medicament. Embodiments 48. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiments 49. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiments 50. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of cancer. Embodiments 51. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. Embodiments 52. The method according to embodiment 51, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or composition according to any one of embodiments 44 to 45. Embodiments 53. The method according to any one of embodiments 51 or 52, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. 3.8 Embodiments Family 43 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 259; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 259; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 259; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 270; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 270; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 270; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 282; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 282; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 282. Embodiment 2. The ISVD according to embodiment 1, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 517. Embodiment 3. The ISVD according to any one of embodiments 1 to 2, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 259; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 270; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 282. Embodiment 4. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 296; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 296; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 296; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 302; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 302; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 302; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 282; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 282; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 282. Embodiment 5. The ISVD according to embodiment 4, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 517. Embodiment 6. The ISVD according to any one of embodiments 4 to 5, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 296; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 302; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 282. Embodiment 7. The ISVD according to any of one embodiments 1 to 6, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 517, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 8. The ISVD according to any one of embodiments 1 to 7, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 9. The ISVD according to any one of embodiments 1 to 8, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 517 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with the amino acid sequence of SEQ ID NOs: 517. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 517 and 523. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 10-8to 10-11moles / litre or less, and preferably 10-9to 5.10-11moles / litre or 10-8to 10-10moles / litre or more preferably 10-9to 10-10moles / litre, as determined by Surface Plasmon Resonance. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-2s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-2s-1and 5.10-6s-1, more preferably between 5.10-3s-1and 5.10-6s-1, such as between 5.10-3s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-7M or lower, more preferably of 5.10-8M or lower, or even of 10-8M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 5.10-8M, such as between 10-10M and 5.10-8M, between 10-10M and 10-8M or between 10-11M and 10-8M. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 19. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 18, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 20. The polypeptide or construct according to embodiment 19, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 557 and 565. Embodiment 21. The polypeptide or construct according to any one of embodiments 19 to 20, in which said one or more linkers are one or more amino acid sequences. Embodiment 22. The polypeptide or construct according to any one of embodiments 19 to 21, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 23. The polypeptide or construct according to any one of embodiments 19 to 22, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 24. The polypeptide or construct according to any one of embodiments 19 to 23, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 25. The polypeptide or construct according to any one of embodiments 19 to 24, which is a multivalent construct. Embodiment 26. The polypeptide or construct according to any one of embodiments 19 to 25, which is a multispecific construct. Embodiment 27. The polypeptide or construct according to any one of embodiments 22 to 26, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 28. The polypeptide or construct according to embodiment 27, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 29. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 30. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 32. The polypeptide or construct according to embodiment 31, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 33. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 34. The polypeptide or construct according to embodiment 33, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 35. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 36. The polypeptide or construct according to anyone of embodiments 19 to 35, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 37. The polypeptide or construct according to any of embodiments 19 to 36, further comprising a C-terminal extension. Embodiment 38. The polypeptide or construct according to embodiment 37, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 39. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38. Embodiment 40. The nucleic acid according to embodiment 39, that is in the form of a genetic construct. Embodiment 41. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 39; and / or that comprises the nucleic acid according to any one of embodiments 39 or 40. Embodiment 42. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 39 or 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or the polypeptide according to any one of embodiments 19 to 38. Embodiment 43. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 18, or at least one polypeptide according to any one of embodiments 19 to 38; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or polypeptide according to any one of embodiments 19 to 38. Embodiment 44. A composition comprising at least one ISVD according to any one of embodiments 1 to 18, at least one polypeptide or construct according to any one of embodiments 19 to 38, or at least one nucleic acid according to any one of embodiments 39 or 40. Embodiment 45. The composition according to embodiment 44, which is a pharmaceutical composition. Embodiment 46. The composition according to any one of embodiments 44 or 45, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 47. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use as a medicament. Embodiment 48. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 49. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 50. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 51. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. Embodiment 52. The method according to embodiment 51, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or composition according to any one of embodiments 44 to 45. Embodiment 53. The method according to any one of embodiments 51 or 52, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. 3.9 Embodiments Family 46 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 260; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 260; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 260; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 271; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 271; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 271; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 283; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 283; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 283. Embodiment 2. The ISVD according to embodiment 1, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 518. Embodiment 3. The ISVD according to any one of embodiments 1 to 2, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 260; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 271; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 283. Embodiment 4. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 297; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 297; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 297; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 303; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 303; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 303; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 283; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 283; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 283. Embodiment 5. The ISVD according to embodiment 4, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 518. Embodiment 6. The ISVD according to any one of embodiments 4 to 5, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 297; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 303; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 283. Embodiment 7. The ISVD according to any of one embodiments 1 to 6, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 518, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 8. The ISVD according to any one of embodiments 1 to 7, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 9. The ISVD according to any one of embodiments 1 to 8, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 518 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with the amino acid sequence of SEQ ID NOs: 518. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 518 and 524. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 10-8to 10-11moles / litre or less, and preferably 10-9to 5.10-11moles / litre or 10-8to 10-10moles / litre or more preferably 10-9to 10-10moles / litre, as determined by Surface Plasmon Resonance. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a koff rate between 10-2s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-2s-1and 5.10-6s-1, more preferably between 5.10-3s-1and 5.10-6s-1, such as between 5.10-3s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-7M or lower, more preferably of 5.10-8M or lower, or even of 10-8M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 5.10-8M, such as between 10-10M and 5.10-8M, between 10-10M and 10-8M or between 10-11M and 10-8M. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 19. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 18, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 20. The polypeptide or construct according to embodiment 19, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such the polypeptide with SEQ ID NO: 558. Embodiment 21. The polypeptide or construct according to any one of embodiments 19 to 20, in which said one or more linkers are one or more amino acid sequences. Embodiment 22. The polypeptide or construct according to any one of embodiments 19 to 21, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 23. The polypeptide or construct according to any one of embodiments 19 to 22, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 24. The polypeptide or construct according to any one of embodiments 19 to 23, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 25. The polypeptide or construct according to any one of embodiments 19 to 24, which is a multivalent construct. Embodiment 26. The polypeptide or construct according to any one of embodiments 19 to 25, which is a multispecific construct. Embodiment 27. The polypeptide or construct according to any one of embodiments 22 to 26, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 28. The polypeptide or construct according to embodiment 27, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 29. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 30. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 32. The polypeptide or construct according to embodiment 31, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 33. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 34. The polypeptide or construct according to embodiment 33, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 35. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 36. The polypeptide or construct according to anyone of embodiments 19 to 35, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 37. The polypeptide or construct according to any of embodiments 19 to 36, further comprising a C-terminal extension. Embodiment 38. The polypeptide or construct according to embodiment 37, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 39. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38. Embodiment 40. The nucleic acid according to embodiment 39, that is in the form of a genetic construct. Embodiment 41. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 39; and / or that comprises the nucleic acid according to any one of embodiments 39 or 40. Embodiment 42. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 39 or 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or the polypeptide according to any one of embodiments 19 to 38. Embodiment 43. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 18, or at least one polypeptide according to any one of embodiments 19 to 38; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or polypeptide according to any one of embodiments 19 to 38. Embodiment 44. A composition comprising at least one ISVD according to any one of embodiments 1 to 18, at least one polypeptide or construct according to any one of embodiments 19 to 38, or at least one nucleic acid according to any one of embodiments 39 or 40. Embodiment 45. The composition according to embodiment 44, which is a pharmaceutical composition. Embodiment 46. The composition according to any one of embodiments 44 or 45, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 47. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use as a medicament. Embodiment 48. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 49. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 50. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 51. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. Embodiment 52. The method according to embodiment 51, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or composition according to any one of embodiments 44 to 45. Embodiment 53. The method according to any one of embodiments 51 or 52, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. 3.10 Embodiments Family 45 Embodiment 1. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 262; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 262; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 262; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 272; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 272; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 272; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 284; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 284; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 284. Embodiment 2. The ISVD according to embodiment 1, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 519. Embodiment 3. The ISVD according to any one of embodiments 1 to 2, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 262; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 272; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 284. Embodiment 4. An immunoglobulin single variable domain (ISVD) specifically binding human PD- L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 298; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 298; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 298; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 304; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 304; f) amino acid sequences that have 6, 5, 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 304; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 284; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 284; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 284. Embodiment 5. The ISVD according to embodiment 4, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 519. Embodiment 6. The ISVD according to any one of embodiments 4 to 5, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 298; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 304; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 284. Embodiment 7. The ISVD according to any of one embodiments 1 to 6, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 519, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1. Embodiment 8. The ISVD according to any one of embodiments 1 to 7, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody. Embodiment 9. The ISVD according to any one of embodiments 1 to 8, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb. Embodiment 10. The ISVD according to any one of embodiments 1 to 9, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 519 or from the group consisting of amino acid sequences that have more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with the amino acid sequence of SEQ ID NOs: 519. Embodiment 11. The ISVD according to any one of embodiments 1 to 10, in which the amino acid sequence is chosen from the group consisting of SEQ ID NOs: 519 and 525. Embodiment 12. The ISVD according to any one of embodiments 1 to 11, wherein the ISVD specifically binds to human PD-L1 with a dissociation constant (KD) of 10-8to 10-11moles / litre or less, and preferably 10-9to 5.10-11moles / litre or 10-8to 10-10moles / litre or more preferably 10-9to 10-10moles / litre, as determined by Surface Plasmon Resonance. Embodiment 13. The ISVD according to any one of embodiments 1 to 12, wherein the ISVD specifically binds to human PD-L1 with a kon-rate of between 105M-1s-1to about 107M-1s-1, preferably between 5.105M-1s-1and 107M-1s-1, more preferably between 106M-1s-1and 107M-1s-1, such as between 106M-1s-1and 5.106M-1s-1, as determined by Surface Plasmon Resonance. Embodiment 14. The ISVD according to any one of embodiments 1 to 13, wherein the ISVD specifically binds to human PD-L1 with a koffrate between 10-2s-1(t1 / 2=0.69 s) and 10-6s-1(providing a near irreversible complex with a t1 / 2 of multiple days), preferably between 10-2s-1and 5.10-6s-1, more preferably between 5.10-3s-1and 5.10-6s-1, such as between 5.10-3s-1and 10-5s-1, as determined by Surface Plasmon Resonance. Embodiment 15. The ISVD according to any one of embodiments 1 to 14, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to other members of the B7 family. Embodiment 16. The ISVD according to any one of embodiments 1 to 15, wherein the ISVD specifically bind to human and cyno PD-L1 and do not bind to PD-L2. Embodiment 17. The ISVD according to any one of embodiments 1 to 16, wherein the ISVD has a potency (EC50 values) in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of 10-7M or lower, more preferably of 5.10-8M or lower, or even of 10-8M or lower. For example, in such TT assay, the immunoglobulin single variable domains of the present technology may have a potency (EC50 values) between 10-11M and 5.10-8M, such as between 10-10M and 5.10-8M, between 10-10M and 10-8M or between 10-11M and 10-8M. Embodiment 18. The ISVD according to any one of embodiments 1 to 17, wherein the ISVD has an efficacy in a TT assay (e.g. as described in the Examples), monitored via IFN-gamma production, of at least 50%, such as at least 60%, 70%, 80%, 90%, 95%. Embodiment 19. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of embodiments 1 to 18, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers. Embodiment 20. The polypeptide or construct according to embodiment 19, in which said one or more other groups, residues, moieties or binding units are amino acid sequences, such as one of the polypeptides selected from SEQ ID NOs: 566 and 573. Embodiment 21. The polypeptide or construct according to any one of embodiments 19 to 20, in which said one or more linkers are one or more amino acid sequences. Embodiment 22. The polypeptide or construct according to any one of embodiments 19 to 21, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences. Embodiment 23. The polypeptide or construct according to any one of embodiments 19 to 22, in which said one or more other groups, residues, moieties or binding units are ISVDs. Embodiment 24. The polypeptide or construct according to any one of embodiments 19 to 23, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs. Embodiment 25. The polypeptide or construct according to any one of embodiments 19 to 24, which is a multivalent construct. Embodiment 26. The polypeptide or construct according to any one of embodiments 19 to 25, which is a multispecific construct. Embodiment 27. The polypeptide or construct according to any one of embodiments 22 to 26, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units. Embodiment 28. The polypeptide or construct according to embodiment 27, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins. Embodiment 29. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof. Embodiment 30. The polypeptide or construct according to embodiment 28, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 31. The polypeptide or construct according to embodiment 30, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG). Embodiment 32. The polypeptide or construct according to embodiment 31, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin. Embodiment 33. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 47. Embodiment 34. The polypeptide or construct according to embodiment 33, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO: 47. Embodiment 35. The polypeptide or construct according to embodiment 32, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42). Embodiment 36. The polypeptide or construct according to anyone of embodiments 19 to 35, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69. Embodiment 37. The polypeptide or construct according to any of embodiments 19 to 36, further comprising a C-terminal extension. Embodiment 38. The polypeptide or construct according to embodiment 37, wherein said C- terminal extension is a C-terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I). Embodiment 39. A nucleic acid that encodes an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38. Embodiment 40. The nucleic acid according to embodiment 39, that is in the form of a genetic construct. Embodiment 41. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 39; and / or that comprises the nucleic acid according to any one of embodiments 39 or 40. Embodiment 42. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, at least comprising the steps of: a) expressing, in a suitable host cell or non-human host organism or in another suitable expression system, a nucleic acid according to any one of embodiments 39 or 40; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or the polypeptide according to any one of embodiments 19 to 38. Embodiment 43. A method for producing an ISVD according to any one of embodiments 1 to 18, or a polypeptide according to any one of embodiments 19 to 38, said method at least comprising the steps of: a) cultivating and / or maintaining a non-human host or host cell according to embodiment 43 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of embodiments 1 to 18, or at least one polypeptide according to any one of embodiments 19 to 38; optionally followed by: b) isolating and / or purifying the ISVD according to any one of embodiments 1 to 18, or polypeptide according to any one of embodiments 19 to 38. Embodiment 44. A composition comprising at least one ISVD according to any one of embodiments 1 to 18, at least one polypeptide or construct according to any one of embodiments 19 to 38, or at least one nucleic acid according to any one of embodiments 39 or 40. Embodiment 45. The composition according to embodiment 44, which is a pharmaceutical composition. Embodiment 46. The composition according to any one of embodiments 44 or 45, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds. Embodiment 47. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use as a medicament. Embodiment 48. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder. Embodiment 49. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved. Embodiment 50. The ISVD according to any one of embodiments 1 to 18, the polypeptide or construct according to any one of embodiments 19 to 38, or the composition according to any one of embodiments 44 to 45, for use in the diagnosis, prevention and / or treatment of cancer. Embodiment 51. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. Embodiment 52. The method according to embodiment 51, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or composition according to any one of embodiments 44 to 45. Embodiment 53. The method according to any one of embodiments 51 or 52, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of embodiments 1 to 18, a polypeptide or construct according to any one of embodiments 19 to 38, or a composition according to any one of embodiments 44 to 45. 4 Brief description of the drawings Figure 1: Off-rates of the unique sequenced anti-PD-L1 clones on human, cyno and mouse PD-L1. Off-rates were measured using Surface Plasmon Resonance on Proteon. (A) dot plot; (B) dot plot showing cross-reactivity. Figure 2: FACS competition profiles of some of the purified anti-PD-L1 VHHs on CHO cells expressing human PD-L1. Competitor is human PD-1. Figure 3: FACS competition profiles of some of the purified anti-PD-L1 VHHs on ES-2 cells. Competitor is human PD-1. Figure 4: FACS competition profiles of different batches of monovalent ISVD10F11 (ISVD86), bivalent ISVD10F11 (ISVD87), bivalent ISVD36E11 (ISVD84) and of Benchmark1 on ES-2 cells, with human PD-1 as competitor. Figure 5: IFN-gamma production in a TT assay with monovalent and bivalent 36E11, 10F11 and 52A01 compared to Benchmark1. Figure 6: IFN-gamma production in the TT assay with monovalent ISVD10F11 (ISVD40) compared to Benchmark5. Figure 7: IFN-gamma production in a TT assay with monovalent ISVD comprising the parental CDR sequence and ISVDs comprising the sequence optimized CDR sequences. (A) A025502469, A025503388 and a bivalent, benchmark; (B) A025502469, A025503357, A025503359; (C) A025501511, A025502467, A02552469; (D) A025501511, A025502470, A02552471. Figure 8: Determination of the structure of the ISVD10F11 / PD-L1 interaction site by Cryogenic electron microscopy (cryo-EM). ISVD10F11 is depicted as the top molecule. The extended loop shown below is CDR3. The loop to the right is CDR2 of ISVD10F11. The molecule depicted at the bottom is PD-L1. 5 Detailed description of the present technology 5.1 Definitions Unless indicated or defined otherwise, all terms used have their usual meaning in the art, which will be clear to the skilled person. Reference is for example made to the standard handbooks, such as Sambrook et al. (Molecular Cloning: A Laboratory Manual (2nd.Ed.) Vols.1-3, Cold Spring Harbor Laboratory Press, 1989), F. Ausubel et al. (Current protocols in molecular biology, Green Publishing and Wiley Interscience, New York, 1987), Lewin (Genes II, John Wiley & Sons, New York, N.Y., 1985), Old et al. (Principles of Gene Manipulation: An Introduction to Genetic Engineering (2nd edition) University of California Press, Berkeley, CA, 1981); Roitt et al. (Immunology (6th. Ed.) Mosby / Elsevier, Edinburgh, 2001), Roitt et al. (Roitt’s Essential Immunology (10th Ed.) Blackwell Publishing, UK, 2001), and Janeway et al. (Immunobiology (6th Ed.) Garland Science Publishing / Churchill Livingstone, New York, 2005), as well as to the general background art cited herein. Unless indicated otherwise, all methods, steps, techniques and manipulations that are not specifically described in detail can be performed and have been performed in a manner known per se, as will be clear to the skilled person. Reference is for example again made to the standard handbooks and the general background art mentioned herein and to the further references cited therein; as well as to for example the following reviews Presta (Adv. Drug Deliv. Rev.58 (5-6): 640- 56, 2006), Levin and Weiss (Mol. Biosyst. 2(1): 49-57, 2006), Irving et al. (J. Immunol. Methods 248(1-2): 31-45, 2001), Schmitz et al. (Placenta 21 Suppl. A: S106-12, 2000), Gonzales et al. (Tumour Biol. 26(1): 31-43, 2005), which describe techniques for protein engineering, such as affinity maturation and other techniques for improving the specificity and other desired properties of proteins such as immunoglobulins. The term “sequence” as used herein (for example in terms like “immunoglobulin sequence”, “antibody sequence”, “variable domain sequence”, “VHH sequence” or “protein sequence”), should generally be understood to include both the relevant amino acid sequence as well as nucleic acids or nucleotide sequences encoding the same, unless the context requires a more limited interpretation. Amino acid residues will be indicated according to the standard three-letter or one-letter amino acid code. Reference is made to Table A-2 on page 48 of WO 08 / 020079. A nucleic acid or amino acid is considered to be “(in) (essentially) isolated (form)” - for example, compared to the reaction medium or cultivation medium from which it has been obtained - when it has been separated from at least one other component with which it is usually associated in said source or medium, such as another nucleic acid, another protein / polypeptide, another biological component or macromolecule or at least one contaminant, impurity or minor component. In particular, a nucleic acid or amino acid is considered “(essentially) isolated” when it has been purified at least 2-fold, in particular at least 10-fold, more in particular at least 100-fold, and up to 1000-fold or more. A nucleic acid or amino acid that is “in (essentially) isolated form” is preferably essentially homogeneous, as determined using a suitable technique, such as a suitable chromatographical technique, such as polyacrylamide-gel electrophoresis. When a nucleotide sequence or amino acid sequence is said to “comprise” another nucleotide sequence or amino acid sequence, respectively, or to “essentially consist of” another nucleotide sequence or amino acid sequence, this may mean that the latter nucleotide sequence or amino acid sequence has been incorporated into the first mentioned nucleotide sequence or amino acid sequence, respectively, but more usually this generally means that the first mentioned nucleotide sequence or amino acid sequence comprises within its sequence a stretch of nucleotides or amino acid residues, respectively, that has the same nucleotide sequence or amino acid sequence, respectively, as the latter sequence, irrespective of how the first mentioned sequence has actually been generated or obtained (which may for example be by any suitable method described herein). By means of a non-limiting example, when a polypeptide is said to comprise an immunoglobulin single variable domain, this may mean that said immunoglobulin single variable domain sequence has been incorporated into the sequence of the polypeptide, but more usually this generally means that the polypeptide contains within its sequence the sequence of the immunoglobulin single variable domains irrespective of how said polypeptide has been generated or obtained. Also, when a nucleic acid or nucleotide sequence is said to comprise another nucleotide sequence, the first mentioned nucleic acid or nucleotide sequence is preferably such that, when it is expressed into an expression product (e.g. a polypeptide), the amino acid sequence encoded by the latter nucleotide sequence forms part of said expression product (in other words, that the latter nucleotide sequence is in the same reading frame as the first mentioned, larger nucleic acid or nucleotide sequence). By “(essentially) consist of” is meant that the later nucleic acid sequence or amino acid sequence, either is exactly the same as the polypeptide (e.g. the CDR region; the ISVD) or corresponds to the polypeptide (e.g. the CDR region; the ISVD) which has a limited number of amino acid residues, such as 1-20 amino acid residues, for example 1-10 amino acid residues and preferably 1-6 amino acid residues, such as 1, 2, 3, 4, 5 or 6 amino acid residues, added at the amino terminal end, at the carboxy terminal end, or at both the amino terminal end and the carboxy terminal end of the immunoglobulin single variable domain. For the purposes of comparing two or more amino acid sequences, the percentage of “sequence identity” between a first amino acid sequence and a second amino acid sequence may be calculated by dividing [the number of amino acid residues in the first amino acid sequence that are identical to the amino acid residues at the corresponding positions in the second amino acid sequence] by [the total number of amino acid residues in the first amino acid sequence] and multiplying by [100%], in which each deletion, insertion, substitution or addition of an amino acid residue in the second amino acid sequence - compared to the first amino acid sequence - is considered as a difference at a single amino acid residue (i.e. at a single position). Usually, for the purpose of determining the percentage of “sequence identity” between two amino acid sequences in accordance with the calculation method outlined hereinabove, the amino acid sequence with the greatest number of amino acid residues will be taken as the “first” amino acid sequence, and the other amino acid sequence will be taken as the “second” amino acid sequence. An “amino acid difference” as used herein refers to a deletion, insertion or substitution of a single amino acid residue vis-à-vis a reference sequence. In one embodiment, an “amino acid difference” is a substitution. In one embodiment, amino acid substitutions are conservative substitutions. Such conservative substitutions are substitutions in which one amino acid within the following groups (a) – (e) is substituted by another amino acid residue within the same group: (a) small aliphatic, nonpolar or slightly polar residues: Ala, Ser, Thr, Pro and Gly; (b) polar, negatively charged residues and their (uncharged) amides: Asp, Asn, Glu and Gln; (c) polar, positively charged residues: His, Arg and Lys; (d) large aliphatic, nonpolar residues: Met, Leu, Ile, Val and Cys; and (e) aromatic residues: Phe, Tyr and Trp. In one embodiment, conservative substitutions are as follows: Ala into Gly or into Ser; Arg into Lys; Asn into Gln or into His; Asp into Glu; Cys into Ser; Gln into Asn; Glu into Asp; Gly into Ala or into Pro; His into Asn or into Gln; Ile into Leu or into Val; Leu into Ile or into Val; Lys into Arg, into Gln or into Glu; Met into Leu, into Tyr or into Ile; Phe into Met, into Leu or into Tyr; Ser into Thr; Thr into Ser; Trp into Tyr; Tyr into Trp; and / or Phe into Val, into Ile or into Leu. A “VHH family” as used in the present specification refers to a group of VHH sequences that have identical lengths (i.e. they have the same number of amino acids within their sequence) and of which the amino acid sequence between position 8 and position 106 (according to Kabat numbering) has an amino acid sequence identity of 89% or more. The terms “epitope” and “antigenic determinant”, which can be used interchangeably, refer to the part of a macromolecule, such as a polypeptide or protein (e.g., PD-L1) that is recognized by antigen-binding molecules, such as immunoglobulins, conventional antibodies, or immunoglobulin single variable domains, and more particularly by the antigen-binding site of said molecules. Epitopes define the minimum binding site for an immunoglobulin, and thus represent the target of specificity of an immunoglobulin. The part of an antigen-binding molecule (such as an immunoglobulin, a conventional antibody, an immunoglobulin single variable domain) that recognizes the epitope is called a “paratope”. The framework regions of a variable domain, such as an immunoglobulin single variable domain form the β sheets that provide the structural framework of the domain, whereas the hypervariable sequences correspond to three loops at the outer edge of the β barrel, which are juxtaposed in the folded domain. These hypervariable loops are brought together, creating a single hypervariable site at the tip of the molecule. This is the binding site for antigen, the antigen-binding site or paratope. The three hypervariable loops determine antigen specificity by forming a surface complementary to the antigen and are more commonly termed the complementarity-determining regions, or CDRs (CDR1, CDR2, and CDR3). As such, it is the surface of the antibody molecule formed by the juxtaposition of the CDRs that creates the site to which an antigen binds. An antibody generally recognizes only a small region on the surface of a large molecule such as a protein. The structure recognized by an antibody is called an antigenic determinant or epitope. Such sites are likely to be composed of amino acids from different parts of the polypeptide chain that have been brought together by protein folding. Antigenic determinants of this kind are known as conformational or discontinuous epitopes because the structure recognized is composed of segments of the protein that are discontinuous in the amino acid sequence of the antigen but are brought together in the three-dimensional structure. In contrast, an epitope composed of a single segment of polypeptide chain is termed a continuous or linear epitope. The binding between an antibody antigen binding site (paratope) and its epitope is a reversible noncovalent interaction. The forces, or bonds, involved in these noncovalent interactions are electrostatic forces, hydrogen bounds, Van der Waals forces and hydrophobic forces. Electrostatic interactions occur between charged amino acid side chains, as in salt bridges. Interactions also occur between electric dipoles, as in hydrogen bonds, or can involve short-range van der Waals forces. Hydrophobic interactions occur when two hydrophobic surfaces come together to exclude water. For some antigens, hydrophobic interactions probably account for most of the binding energy. The contribution of each of these forces to the overall interaction depends on the particular antibody and antigen involved. A striking difference between antibody interactions with protein antigens and most other natural protein-protein interactions is that antibodies possess many aromatic amino acids in their antigen-binding sites. These amino acids participate mainly in van der Waals and hydrophobic interactions, and sometimes in hydrogen bonds. In general, the hydrophobic and van der Waals forces operate over very short ranges and serve to pull together two surfaces that are complementary in shape: hills on one surface must fit into valleys on the other for good binding to occur. In contrast, electrostatic interactions between charged side chains, and hydrogen bonds bridging oxygen and / or nitrogen atoms, accommodate specific features or reactive groups while strengthening the interaction overall. An amino acid that is involved in one of these noncovalent interactions between the antibody antigen binding site (paratope) and its epitope is also said to “form the interaction site” between the antigen binding site (paratope) and the epitope. A polypeptide (such as an immunoglobulin, an antibody, an immunoglobulin single variable domain, or generally an antigen binding molecule or a fragment thereof) that can “bind to” or “specifically bind to”, that “has affinity for” and / or that “has specificity for” a certain epitope, antigen or protein (or for at least one part, fragment or epitope thereof) is said to be "against" or “directed against” said epitope, antigen or protein or is a “binding” molecule with respect to such epitope, antigen or protein, or is said to be “anti”-epitope, “anti”-antigen or “anti”-protein (e.g., “anti”-PD-L1). The terms “specificity”, “binding specifically” or “specific binding” refer to the number of different target molecules, such as antigens, from the same organism to which a particular binding unit, such as an ISVD, can bind with sufficiently high affinity (see below). “Specificity”, “binding specifically” or “specific binding” are used interchangeably herein with “selectivity”, “binding selectively” or “selective binding”. Binding units, such as ISVDs, specifically bind to their designated targets. The specificity / selectivity of a binding unit can be determined based on affinity. The affinity denotes the strength or stability of a molecular interaction. The affinity is commonly given by the KD, or dissociation constant, which has units of mol / liter (or M). The affinity can also be expressed as an association constant, KA, which equals 1 / KD and has units of (mol / liter)-1(or M-1). The affinity is a measure for the binding strength between a moiety and a binding site on the target molecule: the lower the value of the KD, the stronger the binding strength between a target molecule and a targeting moiety. Typically, binding units used in the present technology, such as ISVDs, will bind to their targets with a dissociation constant (KD) of 10-5to 10-12moles / liter or less, 10-7to 10-12moles / liter or less, or 10-8to 10-12moles / liter (i.e. with an association constant (KA) of 105to 1012liter / moles or more, 107to 1012liter / moles or more, or 108to 1012liter / moles). Any KD value greater than 10-4mol / liter (or any KA value lower than 104liters / mol) is generally considered to indicate non-specific binding. The KD for biological interactions, such as the binding of immunoglobulin sequences to an antigen, which are considered specific are typically in the range of 10-5moles / liter (10000 nM or 10µM) to 10-12moles / liter (0.001 nM or 1 pM) or less. Accordingly, specific / selective binding may mean that -using the same measurement method, e.g., SPR- a binding unit (or polypeptide comprising the same) binds to PD-L1 with a KD value of 10-5to 10-12moles / liter or less and binds to related B7 family members with a KD value greater than 10-4moles / liter. An example of a related B7 family member is PD-L2. Thus, in an embodiment of the present technology, the ISVD binds to (human) PD-L1 with a KD value of 10-5to 10-12moles / liter or less and binds to PD-L2 of the same species with a KD value greater than 10-4moles / liter. Specific binding to a certain target from a certain species does not exclude that the binding unit can also specifically bind to the analogous target from a different species. For example, specific binding to human PD-L1 does not exclude that the binding unit or a polypeptide comprising the same can also specifically bind to PD-L1 from cynomolgus monkeys (“cyno”). Specific binding of a binding unit to its designated target can be determined in any suitable manner known per se, including, for example, Scatchard analysis and / or competitive binding assays, such as radioimmunoassays (RIA), enzyme immunoassays (EIA) and sandwich competition assays, and the different variants thereof known per se in the art; as well as the other techniques mentioned further herein. The dissociation constant may be the actual or apparent dissociation constant, as will be clear to the skilled person. Methods for determining the dissociation constant will be clear to the skilled person, and for example include the techniques mentioned herein. In this respect, it will also be clear that it may not be possible to measure dissociation constants of more than 10-4moles / liter or 10-3moles / liter (e.g., of 10-2moles / liter). Optionally, as will also be clear to the skilled person, the (actual or apparent) dissociation constant may be calculated on the basis of the (actual or apparent) association constant (KA), by means of the relationship [KD = 1 / KA]. An amino acid sequence is said to be “cross-reactive” for two different antigens or antigenic determinants (such as e.g., serum albumin from two different species of mammal, such as e.g., human serum albumin and cyno serum albumin, such as e.g., PD-L1 from different species of mammal, such as e.g., human PD-L1, cyno PD-L1 and mouse PD-L1) if it is specific for (as defined herein) these different antigens or antigenic determinants. The terms “block”, “antagonize”, “compete”, “competing” and “-competition” are used interchangeably herein to mean the ability of an immunoglobulin, antibody, immunoglobulin single variable domain, polypeptide or other binding agent to interfere with the binding of another protein, polypeptides, ligand or binding agent to a given target. The extent to which an immunoglobulin, antibody, immunoglobulin single variable domain, polypeptide or other binding agent is able to interfere with the binding of another ligand to the target, and therefore whether it can be said to “block”, can be determined using competition binding assays. Particularly suitable quantitative competitive blocking assays are described in the Examples and include e.g. a fluorescence-activated cell sorting (FACS) binding assay with PD-L1 expressed on cells. The extent of blocking can be measured by the (reduced) channel fluorescence. The following generally describes a suitable FACS assay for determining whether an immunoglobulin, antibody, immunoglobulin single variable domain, polypeptide or other binding agent blocks or is capable of blocking. It will be appreciated that the assay can be used with any of the immunoglobulin single variable domains and polypeptides described herein. The FACS instrument (e.g., FACS Canto; Becton Dickinson) is operated in line with the manufacturer's recommendations. To evaluate the “blocking” or “competition” between two binding agents (such as an immunoglobulin single variable domain and a natural ligand or another binding agent) for binding PD-L1, a FACS competition experiment can be performed using cells (such as e.g. Flp-In™-293 cells) overexpressing human PD-L1 and the parental cells as background cell line. Different detection reagents can be used including e.g., monoclonal ANTI-FLAG® M2 antibody (Sigma-Aldrich, cat# F1804), monoclonal anti-C-myc antibody (Sigma-Aldrich, cat# WH0004609M2), monoclonal ANTI- HIS TAG antibody (Sigma-Aldrich, cat# SAB1305538), each labeled differently. A wide range of fluorophores can be used as labels in flow cytometry (such as e.g PE (R-Phycoerythrin), 7- aminoactinomycin D (7-AAD), Acridine Orange, various forms of Alexa Fluor, Allophycocyanin (APC), AmCyan, Aminocoumarin, APC Cy5, APC Cy7, APC-H7, APC / Alexa Fluor 750, AsRed2, Azami-Green, Azurite, B ODIPY FL C5-ceramide, BCECF-AM, Bis-oxonol DiBAC2(3), BODIPY-FL, Calcein, Calcein AM, Caroxy-H2DCFDA, Cascade Blue, Cascade Yellow, Cell Tracker Green, Cerulean, CFSE, Chromomycin A3, CM-H2DCFDA, Cy2, Cy3, Cy3.5, Cy3B, Cy5, Cy5.5, Cy7, CyPet, DAF-FM DAF-FM diacetate, DAPI, DCFH (2'7'Dichorodihydrofluorescein), DHR, Dihydrocalcein AM, Dihydrorhoadamine, Dihydrothidium, DiLC1(5), DiOC6(3), DiOC7(3), dKeima-Red, DRAQ5, Dronpa-Green, various forms of DsRed dTomato, various forms of DyLight, E.coli BioParticles AF488, E2-Crimson, E2-Orange, EBFP2, ECFP, various forms of eFluor, EGFP, EGFP*, Emerald, eqFP650, eqFP670, ER-Tracker Blue- White DPX, Ethidium Bromide, Express2, EYFP, Fc OxyBurst Green, Fc OxyBurst Green 123, FITC, Fluo-3, Fluo-4, Fluorescein, Fura-2, Fura-Red, GFPuv, H2DCFDA, HcRed1, Hoechst Blue (33258), Hoechst Red (33342), Hydroxycoumarin, HyPer, Indo-1, Indo-1 Blue (Low Ca2+), Indo-1 Violet (High Ca2+), iRFP, J-Red, JC-1, JC-9, Katushka (TurboFP635), Katushka2 Kusabira-Orange, LDS 751, Lissamine Rhodamine B, various forms of Live / Dead, Lucifer yellow, Lucifer Yellow CH, Lyso Tracker Blue, Lyso Tracker Green, Lyso Tracker Red, mAmertrine, Marina Blue, mBanana, mCFP, mCherry, mCitrine, Methoxycoumarin, mHoneyDew, Midoriishi-Cyan, Mithramycin, Mito Tracker Deep Red, Mito Tracker Green, Mito Tracker Orange, Mito Tracker Red, MitoFluor Green, mKate (TagFP635), mKate2, mKeima, mKeima-Red, mKO, mKOk, mNeptune, Monochlorobimane, mOrange, mOrange2, mRaspberry, mPlum, mRFP1, mStrawberry, mTangerine, mTarquoise, mTFP1, mTFP1 (Teal), NBD, OxyBurst Green H2DCFDA, OxyBurst Green H2HFF BSA, Pacific Blue, PE (R- Phycoerythrin), PE Cy5, PE Cy5.5, PE Cy7, PE Texas Red, PE-Cy5 conjugates, PE-Cy7 conjugates, PerCP (Peridinin chlorphyll protein), PerCP Cy5.5, PhiYFP, PhiYFP-m, Propidium Iodide (PI), various forms of Qdot, Red 613, RFP Tomato, Rhod-2, S65A, S65C, S65L, S65T, Singlet Oxygen Sensor Green, Sirius, SNARF, Superfolder GFP, SYTOX Blue, SYTOX Green, SYTOX Orange, T-Sapphire, TagBFP, TagCFP, TagGFP, TagRFP, TagRFP657, TagYFP, tdTomato, Texas Red, Thiazole Orange, TMRE, TMRM, Topaz, TOTO-1, TO-PRO-1, TRITC, TRITC TruRed, TurboFP602, TurboFP635, TurboGFP, TurboRFP, TurboYFP, Venus, Vybrant CycleDye Violet, Wild Type GFP, X-Rhodamin, Y66F, Y66H, Y66W, YOYO-1, YPet, ZsGreen1, ZsYellow1, Zymosan A BioParticles AF488 (see more at: http: / / www.thefcn.org / flow-fluorochromes). Fluorophores, or simply “fluors”, are typically attached to the antibody (e.g., the immunoglobulin single variable domain) that recognizes PD-L1 or to the antibody that is used as detection reagent. Various conjugated antibodies are available, such as (without being limiting) for example antibodies conjugated to Alexa Fluor®, DyLight®, Rhodamine, PE, FITC, and Cy3. Each fluorophore has a characteristic peak excitation and emission wavelength. The combination of labels which can be used will depend on the wavelength of the lamp(s) or laser(s) used to excite the fluorophore and on the detectors available. To evaluate the competition between two test binding agents (termed A and B) for binding to PD- L1, a dilution series of cold (without any label) binding agent A is added to (e.g., 200000) cells together with the labeled binding agent B*. The concentration of binding agent B* in the test mix should be high enough to readily saturate the binding sites on PD-L1 expressed on the cells. The concentration of binding agent B* that saturates the binding sites for that binding agent on PD-L1 expressed on the cells can be determined with a titration series of binding agent B* on the PD-L1 cells and determination of the EC50 value for binding. In order to work at saturating concentration, binding agent B* can be used at 100x the EC50 concentration. After incubation of the cells with the mixture of binding agent A and binding agent B* and cells wash, read out can be performed on a FACS. First a gate is set on the intact cells as determined from the scatter profile and the total amount of channel fluorescence is recorded. A separate solution of binding agent B* is also prepared. Binding agent B* in this solution should be in the same buffer and at the same concentration as in the test mix (with binding agent A and B*). This separate solution is also added to the cells. After incubation and cells wash, read out can be performed on a FACS. First a gate is set on the intact cells as determined from the scatter profile and the total amount of channel fluorescence is recorded. A reduction of fluorescence for the cells incubated with the mixture of binding agent A and B* compared to the fluorescence for the cells incubated with the separate solution of binding agent B* indicates that binding agent A blocks binding by binding agent B* to PD-L1 expressed on the cells. A cross-blocking immunoglobulin, antibody, immunoglobulin single variable domain, polypeptide or other binding agent is one which will bind to the PD-L1 in the above competition FACS such that during the assay and in the presence of the second binding agent the recorded fluorescence is between 80% and 0.1% (e.g.80% to 4%) of the maximum fluorescence (measured for the separate labelled immunoglobulin, antibody, immunoglobulin single variable domain, polypeptide or other binding agent), specifically between 75% and 0.1% (e.g.75% to 4%) of the maximum fluorescence, and more specifically between 70% and 0.1% (e.g. 70% to 4%) of maximum fluorescence (as just defined above). The competition between two test binding agents (termed A* and B*) for binding to PD-L1 can also be evaluated by adding both binding agents, each labeled with a different fluorophore, to the PD- L1 expressing cells. After incubation and cells wash, read out can be performed on a FACS. A gate is set for each fluorophore and the total amount of channel fluorescence is recorded. Reduction and / or absence of fluorescence of one of the fluorophores indicates blocking by the binding agents for binding to PD-L1 expressed on the cells. Other methods for determining whether an immunoglobulin, antibody, immunoglobulin single variable domain, polypeptide or other binding agent directed against a target blocks, is capable of blocking, competitively binds or is competitive as defined herein are described e.g., in Xiao-Chi Jia et al. (Journal of Immunological Methods 288: 91–98, 2004), Miller et al. (Journal of Immunological Methods 365: 118–125, 2011). As used herein, the term "potency" is a measure of the biological activity of an agent, such as an ISVD or polypeptide. Potency of an agent can be determined by any suitable method known in the art, such as for instance as described in the experimental section. Cell culture-based potency assays are often the preferred format for determining biological activity since they measure the physiological response elicited by the agent and can generate results within a relatively short period of time. Various types of cell-based assays, based on the mechanism of action of the product, can be used, such as e.g., a Tetanus-toxoid autogenic antigen-specific co-culture assay or the NFAT reporter assay (as further described in the Example section). In contrast, the “efficacy” of an agent, such as an ISVD or polypeptide, measures the maximum strength of the effect itself, at saturating agent concentrations. Efficacy indicates the maximum response achievable from the agent. It refers to the ability of the agent to produce the desired (therapeutic) effect. The efficacy of an agent can be evaluated using in vitro functional assays or in vivo models. 5.2 Immunoglobulin single variable domains The present technology aims at providing a novel type of drug for treating diseases associated with PD-L1. The present inventors have surprisingly found that an immunoglobulin single variable domain (ISVD)comprising a CDR1 consisting of the amino acid sequence of SEQ ID NO: 11; a CDR2 consisting of the amino acid sequence of SEQ ID NO: 13 or 14; and a CDR3 consisting of the amino acid sequence of SEQ ID NO: 17 can be used for more potent and efficient blocking of the PD-L / PD- 1 interaction and treatment of diseases associated with PD-L1. The inventors determined the site on PD-L1 that is involved in the interaction between the ISVD and PD-L of which the blocking resulted in a very efficient and potent inhibition of the PD1 / PD-L1 signaling. Blocking of this interaction site was sufficient to inhibition of the PD1 / PD-L1 signaling. Moreover, this interaction site on PD-L1 or epitope could be block by a single variable domain. Surprisingly, the inventors determined that only 2 CDRs of the immunoglobulin single variable domain are involved in this interaction with PD-L1 and the blocking of the PD / PD-L1 interaction. As such, the inventors observed that only 2 CDRs (CDR2 and CD3) of the immunoglobulin variable domain were sufficient for the blocking of the PD / PD-L1 interaction. The term “immunoglobulin single variable domain” (ISVD), interchangeably used with “single variable domain”, defines immunoglobulin molecules wherein the antigen binding site is present on, and formed by, a single immunoglobulin domain. This sets immunoglobulin single variable domains apart from “conventional” immunoglobulins (e.g., monoclonal antibodies) or their fragments (such as Fab, Fab’, F(ab’)2, scFv, di-scFv), wherein two immunoglobulin domains, in particular two variable domains, interact to form an antigen binding site. Typically, in conventional immunoglobulins, a heavy chain variable domain (VH) and a light chain variable domain (VL) interact to form an antigen binding site. In this case, the complementarity determining regions (CDRs) of both VH and VL will contribute to the antigen binding site, i.e., a total of 6 CDRs will be involved in antigen binding site formation. In view of the above definition, the antigen-binding domain of a conventional 4-chain antibody (such as an IgG, IgM, IgA, IgD or IgE molecule; known in the art) or of a Fab fragment, a F(ab')2 fragment, an Fv fragment such as a disulfide linked Fv or a scFv fragment, or a diabody (all known in the art) derived from such conventional 4-chain antibody, would normally not be regarded as an immunoglobulin single variable domain, as, in these cases, binding to the respective epitope of an antigen would normally not occur by one (single) immunoglobulin domain but by a pair of (associating) immunoglobulin domains such as light and heavy chain variable domains, i.e., by a VH- VL pair of immunoglobulin domains, which jointly bind to an epitope of the respective antigen. In contrast, immunoglobulin single variable domains are capable of specifically binding to an epitope of the antigen without pairing with an additional immunoglobulin variable domain. The binding site of an immunoglobulin single variable domain is formed by a single VH, a single VHH or single VL domain. As such, the single variable domain may be a light chain variable domain sequence (e.g., a VL- sequence) or a suitable fragment thereof; or a heavy chain variable domain sequence (e.g., a VH- sequence or VHH sequence) or a suitable fragment thereof; as long as it is capable of forming a single antigen binding unit (i.e., a functional antigen binding unit that essentially consists of the single variable domain, such that the single antigen binding domain does not need to interact with another variable domain to form a functional antigen binding unit). An immunoglobulin single variable domain (ISVD) can for example be a heavy-chain ISVD, such as a VHH, including a humanized VHH, a VH, including a camelized VH and a human VH. In one embodiment, it is a VHH, a camelized VH or humanized VHH. Heavy chain ISVDs can be derived from a conventional four-chain antibody or from a heavy chain antibody. For example, the immunoglobulin single variable domain may be a single domain antibody (or an amino acid sequence that is suitable for use as a single domain antibody), a "dAb" or dAb (or an amino acid sequence that is suitable for use as a dAb) or a Nanobody® ISVD (as defined herein, and including but not limited to a VHH); other single variable domains, or any suitable fragment of any one thereof. In particular, the immunoglobulin single variable domain may be a NANOBODY® ISVD (such as a VHH, including a humanized VHH or camelized VH) or a suitable fragment thereof. [Note: NANOBODY ® and NANOBODIES® are registered trademarks of Ablynx N.V.] “VHH domains”, also known as VHHs, VHH antibody fragments, have originally been described as the antigen binding immunoglobulin variable domain of “heavy chain antibodies” (i.e., of “antibodies devoid of light chains”; Hamers-Casterman et al. Nature 363: 446-448, 1993). The term “VHH domain” has been chosen in order to distinguish these variable domains from the heavy chain variable domains that are present in conventional 4-chain antibodies (which are referred to herein as “VH domains”) and from the light chain variable domains that are present in conventional 4- chain antibodies (which are referred to herein as “VL domains”). For a further description of VHH’s, reference is made to the review article by Muyldermans (Reviews in Molecular Biotechnology 74: 277-302, 2001). The generation of immunoglobulin sequences, such as VHHs, has been described extensively in various publications, among which WO 94 / 04678, Hamers-Casterman et al.1993 and Muyldermans et al. 2001 (Reviews in Molecular Biotechnology 74: 277-302, 2001). In these methods, camelids are immunized with the target antigen in order to induce an immune response against said target antigen. The repertoire of VHHs obtained from said immunization is further screened for VHHs that bind the target antigen. In these instances, the generation of antibodies requires purified antigen for immunization and / or screening. Antigens can be purified from natural sources, or in the course of recombinant production. Immunization and / or screening for immunoglobulin sequences can be performed using peptide fragments of such antigens. Immunoglobulin sequences of different origin, comprising mouse, rat, rabbit, donkey, human and camelid immunoglobulin sequences can be used in the present technology. Also, fully human, humanized or chimeric sequences can be used. For example, camelid immunoglobulin sequences and humanized camelid immunoglobulin sequences, or camelized domain antibodies, e.g., camelized dAb as described by Ward et al (see for example WO 94 / 04678 and Riechmann, Febs Lett., 339:285-290, 1994 and Prot. Eng., 9:531-537, 1996) can be used herein. The ISVDs can be fused forming a multivalent and / or multispecific construct (for multivalent and multispecific polypeptides containing one or more VHH domains and their preparation, reference is also made to Conrath et al. 2001 (J. Biol. Chem., Vol. 276, 10. 7346-7350), as well as to for example WO 96 / 34103 and WO 99 / 23221). However, it should be noted that the ISVD comprised in the present technology is not limited as to the origin of the ISVD sequence (or of the nucleotide sequence used to express it), nor as to the way that the ISVD sequence or nucleotide sequence is (or has been) generated or obtained. Thus, the ISVD sequences may be naturally occurring sequences (from any suitable species) or synthetic or semi-synthetic sequences. In a specific but non-limiting aspect, the ISVD sequence is a naturally occurring sequence (from any suitable species) or a synthetic or semi-synthetic sequence, including but not limited to “humanized” (as defined herein) immunoglobulin sequences (such as partially or fully humanized camelid, mouse or rabbit immunoglobulin sequences, and in particular partially or fully humanized VHH sequences), “camelized” (as defined herein) immunoglobulin sequences (and in particular camelized VH sequences), as well as ISVDs that have been obtained by techniques such as affinity maturation (for example, starting from synthetic, random or naturally occurring immunoglobulin sequences), CDR grafting, veneering, combining fragments derived from different immunoglobulin sequences, PCR assembly using overlapping primers, and similar techniques for engineering immunoglobulin sequences well known to the skilled person; or any suitable combination of any of the foregoing. Similarly, nucleotide sequences may be naturally occurring nucleotide sequences or synthetic or semi-synthetic sequences, and may for example be sequences that are isolated by PCR from a suitable naturally occurring template (e.g. DNA or RNA isolated from a cell), nucleotide sequences that have been isolated from a library (and in particular, an expression library), nucleotide sequences that have been prepared by introducing mutations into a naturally occurring nucleotide sequence (using any suitable technique known per se, such as mismatch PCR), nucleotide sequence that have been prepared by PCR using overlapping primers, or nucleotide sequences that have been prepared using techniques for DNA synthesis known per se. A “humanized VHH” comprises an amino acid sequence that corresponds to the amino acid sequence of a naturally occurring VHH domain, but that has been “humanized” , i.e. by replacing one or more amino acid residues in the amino acid sequence of said naturally occurring VHH sequence (and in particular in the framework sequences) by one or more of the amino acid residues that occur at the corresponding positio...

Claims

CLAIMS 1. An immunoglobulin single variable domain (ISVD) specifically binding to human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), that interacts with one or more amino acids of the PD-L1 protein selected from Y56, E58, E60, D62, N63, Q66, H78 and R113.

2. The immunoglobulin single variable domain (ISVD) of embodiments 1, in which on or more of S51, S53, and N56 (Kabat numbering) in CDR2 and / or A98, A99, A100, A100a, I100d, and I100g (Kabat numbering) in CDR3 form the interaction site with the epitope on the PD-L1 protein.

3. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (AbM numbering) consists of an amino acid sequence X1X2SX3SX4X5NX6X7(SEQ ID NO: 600), wherein X1, X2, X3, X4, X5, X6, and X7are any amino acid independently chosen. and - CDR3 (AbM numbering) consists of an amino acid sequence X1X2X3AAAAX4X5IX6X7IX8X9X10 (SEQ ID NO: 603), wherein X1, X2, X3, X4, X5, X6, X7, X8, X9 and X10 are any amino acid independently chosen.

4. The immunoglobulin single variable domain (ISVD) according to claim 3, wherein - CDR2 (AbM numbering) consists of an amino acid sequence X1X2SX3SX4X5NX6X7(SEQ ID NO: 600), wherein X1 is selected from A, S, T, P and G, X2 is selected from M, L, I, V and C, X3 is selected from A, S, T, P and G, X4 is selected from A, S, T, P and G, X5 is selected from R, H and K, X6 is selected from A, S, T, P and G, and X7 is selected from N and Q; and - CDR3 (AbM numbering) consists of an amino acid sequence X1X2X3AAAAX4X5IX6X7IX8X9X10(SEQ ID NO: 603), wherein X1is selected from A, S, T, P and G, X2is selected from A, S, T, P and G, X3is selected from F, Y, and W, X4is selected from A, S, T, P and G, X5is selected from A, S, T, P and G, X6is selected from A, S, T, P and G, X7 is selected from N, Q, I, L, M, V and C, X8 is selected from F, Y, and W, X9 is selected from D and E and X10 is selected from F, Y, and W.

5. The immunoglobulin single variable domain (ISVD) according to any of claim 3 or 4, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 13 or 14; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 13 or 14; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 13 or 14; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NOs: 17 or 109; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; f) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 109.

6. The immunoglobulin single variable domain (ISVD) according to any of claims 3 to 5, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3is selected from N, T and F; b) amino acid sequences that have at least 80% amino acid identity with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1is selected from A and G, the amino acid residue X2is selected from S and T, and the amino acid residue X3 is selected from N, T and F; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3is selected from N, T and F;and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, the amino acid residue X3is selected from A, P and S, and the amino acid residue X4is selected from N and I; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, and the amino acid residue X3 is selected from N and I; a) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1is selected from A and P, the amino acid residue X2is selected from I, L and T, and the amino acid residue X3 is selected from N and I.

7. The ISVD according to any of claims 3 to 6, in which the amino acid sequences of the CDR1 and CDR2 (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDR1 and CDR2 of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324.

8. The ISVD according to any one of claims 3 to 17, in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 111, or in which ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 104; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

112.

9. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 23 or 24; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 23 or 24; c) amino acid sequences that have 6, 5, 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 23 or 24; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NOs: 17 or 109; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; f) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 39.

10. The immunoglobulin single variable domain (ISVD) according to claim 9, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR2 (Kabat numbering) consists of an amino acid sequence selected from:a) the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6 (SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5is selected from E and K and wherein the amino acid residue X6is selected from G, V and A; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6 (SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4is selected from S and P, wherein the amino acid residue X5is selected from E and K and wherein the amino acid residue X6is selected from G, V and A; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6 (SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5is selected from E and K and wherein the amino acid residue X6is selected from G, V and A; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2 is selected from I, L and T, wherein the amino acid residue X3is selected from A, P and S, and wherein the amino acid residue X4is selected from I and N; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2 is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acidresidue X1is selected from A and P, wherein the amino acid residue X2 is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N.

11. The ISVD according to any one of claims 9 or 10, in which the amino acid sequences of the CDR2 and CDR3 (Kabat numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDR2 and CD3 of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324.

12. The ISVD according to any one of claims 9 to 11, in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 23; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17), or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 122; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 123; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 111.

13. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 11 or 98; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 11 or 98; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 11 or 98; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 13 or 14; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 13 or 14; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 13 or 14; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 17 or 109; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 109.

14. The immunoglobulin single variable domain (ISVD) according to claim 13, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequenceGRX1FSX2NX3MG(SEQ ID NO: 596), wherein the amino acid residue X1 is selected from T, K and G, the amino acid residue X2 is selected from G and R, and the amino acid residue X3 is selected from T and A; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence GRX1FSX2NX3MG (SEQ ID NO: 596), wherein the amino acid residueX1 is selected from T, K and G, the amino acid residue X2 is selected from G and R, and the amino acid residue X3 is selected from T and A; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence GRX1FSX2NX3MG (SEQ ID NO: 596), wherein the amino acid residue X1is selected from T, K and G, the amino acid residue X2is selected from G and R, and the amino acid residue X3is selected from T and A; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3is selected from N, T and F; e) amino acid sequences that have at least 80% amino acid identity with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3 is selected from N, T and F; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the acid sequence of X1ISSX2GRX3TN (SEQ ID NO: 598), wherein the amino acid residue X1 is selected from A and G, the amino acid residue X2 is selected from S and T, and the amino acid residue X3is selected from N, T and F; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, the amino acid residue X3 is selected from A, P and S, and the amino acid residue X4is selected from N and I; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1is selected from A and P, the amino acid residue X2is selected from I, L and T, and the amino acid residue X3 is selected from N and I; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601), wherein the amino acid residue X1 is selected from A and P, the amino acid residue X2 is selected from I, L and T, and the amino acid residue X3is selected from N and I.

15. The ISVD according to any one of clams 13 or 14, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324.

16. The ISVD according to any one of claims 13 to 15, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 11; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1 consists of the amino acid sequence of SEQ ID NO: 11; ^ CDR2 consists of one of the amino acid sequences of SEQ ID NOs: 13 and 14; and ^ CDR3 consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 98; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 98; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 99; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 99; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 11; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 14; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 100; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 103; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 100; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 13; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 111, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 101; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 104; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

112.

17. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 22; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 22; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 22; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 23 or 24; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 23 or 24; f) amino acid sequences that have 6, 5, 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 23 or 24; and- CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 17 or 109; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 17 or 109; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 17 or 109.

18. The immunoglobulin single variable domain (ISVD) according to claim 17, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of X1NX2MG (SEQ ID NO: 604); wherein the amino acid residue X1is selected from G and R and wherein the amino acid residue X2is selected from T and A; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1NX2MG (SEQ ID NO: 604); wherein the amino acid residue X1 is selected from G and R and wherein the amino acid residue X2 is selected from T and A; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence X1NX2MG (SEQ ID NO: 604); wherein the amino acid residue X1is selected from G and R and wherein the amino acid residue X2 is selected from T and A; and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6(SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from S and T, wherein the amino acid residue X3is selected from T, N and F, wherein the amino acid residue X4is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6 (SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2isselected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1ISSX2GRX3TNYADX4VX5X6(SEQ ID NO: 605), wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from S and T, wherein the amino acid residue X3 is selected from T, N and F, wherein the amino acid residue X4 is selected from S and P, wherein the amino acid residue X5 is selected from E and K and wherein the amino acid residue X6 is selected from G, V and A; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2 is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2is selected from I, L and T, wherein the amino acid residue X3 is selected from A, P and S, and wherein the amino acid residue X4 is selected from I and N; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SSWAAAX1GTX2X3X4IYDY (SEQ ID NO: 601); wherein the amino acid residue X1is selected from A and P, wherein the amino acid residue X2is selected from I, L and T, wherein the amino acid residue X3is selected from A, P and S, and wherein the amino acid residue X4is selected from I and N.

19. The ISVD according to any one of claims 17 or 18, in which the amino acid sequences of the CDRs (Kabat numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequenceidentity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NO: 4, SEQ ID NO: 5 and SEQ ID NOs: 311-324.

20. The ISVD according to any one of claims 17 to 19, in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2 (Kabat numbering) consists of one of the amino acid sequences of SEQ ID NO: 23 and 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 23; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:22; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 24; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 109, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 22; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 120; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 17, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 118; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 122; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 110, or in which^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 118; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 123; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 111, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 118; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 123; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

111.

21. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 132; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 132; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 132; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 134 or 136; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 134 or 136; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 134 or 136; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 143 or 145; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 143 or 145; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 143 or 145.

22. The immunoglobulin single variable domain (ISVD) according to claim 21, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence GGTFRHYVMG (SEQ ID NO: 132); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence GGTFRHYVMG (SEQ ID NO: 132); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence GGTFRHYVMG (SEQ ID NO: 132); and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1X2SWSGSGX3Y (SEQ ID NO: 607), wherein the amino acid residue X1is selected from A or G, the amino acid residue X2is selected from I, S and V, and the amino acid residue X3 is selected from T and Q; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1X2SWSGSGX3Y (SEQ ID NO: 607), wherein the amino acid residue X1 is selected from A or G, the amino acid residue X2 is selected from I, S and V, and the amino acid residue X3 is selected from T and Q; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1X2SWSGSGX3Y (SEQ ID NO: 607), wherein the amino acid residue X1 is selected from A or G, the amino acid residue X2 is selected from I, S and V, and the amino acid residue X3 is selected from T and Q; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608), wherein the amino acid residue X1is selected from A, V, R, K and E, the amino acid residue X2is selected from Q and G, the amino acid residue X3is selected from V, K, Q, H and E, and the amino acid residue X4is selected from L and R; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608), wherein the amino acid residue X1 is selected from A, V, R, K and E, the amino acid residue X2 is selected from Q and G, the amino acid residue X3 is selected from V, K, Q, H and E, and the amino acid residue X4is selected from L and R;;i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608), wherein the amino acid residue X1 is selected from A, V, R, K and E, the amino acid residue X2 is selected from Q and G, the amino acid residue X3 is selected from V, K, Q, H and E, and the amino acid residue X4is selected from L and R.

23. The ISVD according to any one of claims 21 or 22, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 340-379.

24. The ISVD according to any one of claims 21 to 23, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 134; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 143, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 147,or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 148, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 149, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 150, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 149, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 149,or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 150, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 136; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 135; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 151, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 132; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 137; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

152.

25. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 159; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 159; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 159; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 160 or 162; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 160 or 162; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 160 or 162;and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 143 or 145; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 143 or 145; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 143 or 145.

26. The immunoglobulin single variable domain (ISVD) according to claim 25, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of HYVMG (SEQ ID NO: 159); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of HYVMG (SEQ ID NO: 159); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of HYVMG (SEQ ID NO: 159); and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1X2SWSGSGX3YYADSVKG (SEQ ID NO: 610); wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from I, S and V, and wherein the amino acid residue X3 is selected from T and Q; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1X2SWSGSGX3YYADSVKG (SEQ ID NO: 610); wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2is selected from I, S and V, and wherein the amino acid residue X3is selected from T and Q; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1X2SWSGSGX3YYADSVKG (SEQ ID NO: 610); wherein the amino acid residue X1is selected from G and A, wherein the amino acid residue X2 is selected from I, S and V, and wherein the amino acid residue X3 is selected from T and Q;and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608); wherein the amino acid residue X1is selected from A, V, R, K or E, wherein the amino acid residue X2is selected from Q or G, wherein the amino acid residue X3is selected from V, K, Q, H or E, and wherein the amino acid residue X4is selected from R or L; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608); wherein the amino acid residue X1is selected from A, V, R, K or E, wherein the amino acid residue X2 is selected from Q or G, wherein the amino acid residue X3 is selected from V, K, Q, H or E, and wherein the amino acid residue X4is selected from R or L; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of DX1TSX2VX3X4AVEAYDY (SEQ ID NO: 608); wherein the amino acid residue X1is selected from A, V, R, K or E, wherein the amino acid residue X2 is selected from Q or G, wherein the amino acid residue X3 is selected from V, K, Q, H or E, and wherein the amino acid residue X4 is selected from R or L.

27. The ISVD according to claims 25 or 26, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 340-379.

28. The ISVD according to any one of claims 25 to 27, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 160; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 143, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 148, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 149, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

150. or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 148, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 144, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 146, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 166; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 145, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

149. or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 147, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 162; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 150, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 161; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 151, or in which^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 159; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 163; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

152.

29. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 169; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 169; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 169; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 171 or 175; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 171 or 175; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 171 or 175; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 192 or 193; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 192 or 193; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 192 or 193.

30. The immunoglobulin single variable domain (ISVD) according to claim 29, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence GSIFTSAVME (SEQ ID NO: 169);b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence GSIFTSAVME (SEQ ID NO: 169); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence GSIFTSAVME (SEQ ID NO: 169); and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of RIGSGGRX1A (SEQ ID NO: 611), wherein the amino acid residue X1 is selected from I, R, H, E, K, T, Q, A, S,P, and G; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RIGSGGRX1A (SEQ ID NO: 611), wherein the amino acid residue X1 is selected from I, R, H, E, K, T, Q, A, S,P, and G; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RIGSGGRX1A (SEQ ID NO: 611), wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q, A, S,P, and G; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of VX1GGYIY (SEQ ID NO: 612), wherein the amino acid residue X1 is selected from M and I; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612), wherein the amino acid residue X1is selected from M and I; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612), wherein the amino acid residue X1 is selected from M and I.

31. The ISVD according to any one of claims 29 or 30, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 420-450.

32. The ISVD according to any one of claims 29 to 30, in which^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 171; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 192, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 172; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 173; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 174; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 175; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 176; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 177; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 178; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 171; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 179; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 180; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 169; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 181; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

193.

33. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 197; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 197; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 197; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 198 or 202; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 198 or 202; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 198 or 202; and- CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 192 or 193; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 192 or 193; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 192 or 193.

34. The immunoglobulin single variable domain (ISVD) according to claim 33, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SAVME (SEQ ID NO: 197); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SAVME (SEQ ID NO: 197); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SAVME (SEQ ID NO: 197); and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of RIGSGGRX1AYX2DSVKG (SEQ ID NO: 613); wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q; A, S, P and G, and wherein the amino acid residue X2 is selected from P and V; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RIGSGGRX1AYX2DSVKG (SEQ ID NO: 613); wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q; A, S, P and G, and wherein the amino acid residue X2is selected from P and V; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RIGSGGRX1AYX2DSVKG (SEQ ID NO: 613); wherein the amino acid residue X1is selected from I, R, H, E, K, T, Q; A, S, P and G, and wherein the amino acid residue X2 is selected from P and V; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of VX1GGYIY (SEQ ID NO: 612); wherein the amino acid residue X1is selected from M and I;h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612); wherein the amino acid residue X1is selected from M and I; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of VX1GGYIY (SEQ ID NO: 612); wherein the amino acid residue X1is selected from M and I.

35. The ISVD according to any one of claims 33 or 34, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 420-450.

36. The ISVD according to any one of claims 33 to 35, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 198; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 192, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 199; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 200; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 201; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 202; and^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 203; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 204; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 205; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

193. or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 198; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 206; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 207; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 208; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 193, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 197; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 209; and^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

193.

37. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 218; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 218; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 218; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 222; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 222; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 222; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 234; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 234; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

234.

38. The immunoglobulin single variable domain (ISVD) according to claim 37, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence ERTFX1SYSMG (SEQ ID NO: 617), wherein the amino acid residue X1 is selected from S and G;b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence ERTFX1SYSMG (SEQ ID NO: 617), wherein the amino acid residue X1 is selected from S and G; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence ERTFX1SYSMG (SEQ ID NO: 617), wherein the amino acid residue X1is selected from S and G; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1IX2YRGAGPY (SEQ ID NO: 614), wherein the amino acid residue X1 is selected from V and P, and wherein the amino acid residue X2 is selected from G and F; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1IX2YRGAGPY (SEQ ID NO: 614), wherein the amino acid residue X1 is selected from V and P, and wherein the amino acid residue X2 is selected from G and F; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1IX2YRGAGPY (SEQ ID NO: 614), wherein the amino acid residue X1 is selected from V and P, and wherein the amino acid residue X2 is selected from G and F; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615), wherein the amino acid residue X1 is selected from A and T; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615), wherein the amino acid residue X1is selected from A and T; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615), wherein the amino acid residue X1 is selected from A and T.

39. The ISVD according to any one of claims 37 or 38, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99%amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 482-497.

40. The ISVD according to any one of claims 37 to 40, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 218; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 222; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 218; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 223; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 218; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 224; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 219; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 222; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

235.

41. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 241; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 241; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 241; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 242;e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 242; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 242; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 234; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 234; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

234.

42. The immunoglobulin single variable domain (ISVD) according to claim 41, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SYSMG (SEQ ID NO: 241); b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SYSMG (SEQ ID NO: 241); c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SYSMG (SEQ ID NO: 241); and - CDR2 (Kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of X1IX2YRGAGPYYEDSVKG (SEQ ID NO: 616); wherein the amino acid residue X1is selected V and P, and wherein the amino acid residue X2is selected from G and F; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of X1IX2YRGAGPYYEDSVKG (SEQ ID NO: 616); wherein the amino acid residue X1is selected V and P, and wherein the amino acid residue X2 is selected from G and F; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of X1IX2YRGAGPYYEDSVKG (SEQ ID NO: 616); wherein the amino acidresidue X1is selected V and P, and wherein the amino acid residue X2 is selected from G and F; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615); wherein the amino acid residue X1is selected from A and T; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615); wherein the amino acid residue X1is selected from A and T; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of RDGAVASIPQAFX1S (SEQ ID NO: 615); wherein the amino acid residue X1is selected from A and T.

43. The ISVD according to any one of clams 41 or 42, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 482-497.

44. The ISVD according to any one of claims 41 to 43, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 242; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 243; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 244; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 234, or in which^ CDR1 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 241; ^ CDR2 (Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 242; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

235.

45. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 256; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 256; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 256; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 267; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 267; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 267; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 279; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 279; i) amino acid sequences that have 43, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

279.

46. The ISVD according to claim 45, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with theamino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

514.

47. The ISVD according to any one of claims 45 or 46, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 256; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 267; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

279.

48. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 293; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 293; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 293; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 299; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 299; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 299; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 279; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 279; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 279.

49. The ISVD according to claim 48, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

514.

50. The ISVD according to any one of claims 48 or 49, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 293; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 299; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

279.

51. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 257; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 257; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 257; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 268; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 268; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 268; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 280; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 280;i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

280.

52. The ISVD according to claim 51, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

515.

53. The ISVD according to any one of claims 51 or 52, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 257; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 268; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

280.

54. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 294; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 294; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 294; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 300; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 300; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 300; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from:g) the amino acid sequence of SEQ ID NOs: 280; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 280; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

280.

55. The ISVD according to claim 54, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

515.

56. The ISVD according to any one of claims 54 or 55, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 294; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 300; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

280.

57. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 258; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 258; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 258; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 269; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 269;f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 269; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 281; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 281; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

281.

58. The ISVD according to claim 57, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

516.

59. The ISVD according to any one of claims 57 to 58, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 258; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 269; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

281.

60. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 295; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 295; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 295; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from:d) the amino acid sequence of SEQ ID NO: 301; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 301; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 301; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 281; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 281; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

281.

61. The ISVD according to claim 60, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

516.

62. The ISVD according to any one of claims 60 or 61 in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 295; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 301; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

281.

63. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 259; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 259;c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 259; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 270; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 270; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 270; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 282; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 282; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

282.

64. The ISVD according to claim 63, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

517.

65. The ISVD according to any one of claims 63 or 64, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 259; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 270; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

282.

66. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from:a) the amino acid sequence of SEQ ID NO: 296; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 296; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 296; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 302; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 302; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 302; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 282; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 282; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

282.

67. The ISVD according to claim 66, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

517.

68. The ISVD according to any one of claims 66 or 67, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 296; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 302; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 282.

69. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 260; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 260; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 260; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 271; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 271; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 271; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 283; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 283; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

283.

70. The ISVD according to claim 69, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

518.

71. The ISVD according to any one of claims 69 or 70, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 260;^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 271; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

283.

72. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 297; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 297; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 297; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 303; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 303; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 303; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 283; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 283; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

283.

73. The ISVD according to claim 72, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs: 518.

74. The ISVD according to any one of claims 72 or 73, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 297; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 303; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

283.

75. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (AbM numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 262; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 262; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 262; and - CDR2 (AbM numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 272; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 272; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 272; and - CDR3 (AbM numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 284; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 284; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

284.

76. The ISVD according to claim 75, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with theamino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

519.

77. The ISVD according to any one of claims 75 or 76, in which ^ CDR1 (AbM numbering) consists of the amino acid sequence of SEQ ID NO: 262; ^ CDR2 (AbM numbering) consists of the amino acid sequences of SEQ ID NO: 272; and ^ CDR3 (AbM numbering) consists of the amino acid sequence of SEQ ID NO:

284.

78. An immunoglobulin single variable domain (ISVD) specifically binding human PD-L1, that essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3, respectively), in which: - CDR1 (Kabat numbering) consists of an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 298; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 298; c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 298; and - CDR2 (kabat numbering) consists of an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 304; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 304; f) amino acid sequences that have 6, 5, 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 304; and - CDR3 (Kabat numbering) consists of an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 284; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 284; i) amino acid sequences that have 4, 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 284.

79. The ISVD according to claim 78, in which the amino acid sequences of the CDRs (AbM numbering) have at least 80% amino acid sequence identity, more preferably at least 90% amino acid sequence identity, such as 95% amino acid sequence identity or 99% amino acid sequence identity or more, or even essentially 100% amino acid sequence identity with the amino acid sequences of the CDRs of the ISVD with the amino acid sequence selected from SEQ ID NOs:

519.

80. The ISVD according to any one of claims 78 or 79, in which ^ CDR1(Kabat numbering) consists of the amino acid sequence of SEQ ID NO: 298; ^ CDR2(Kabat numbering) consists of the amino acid sequences of SEQ ID NO: 304; and ^ CDR3 (Kabat numbering) consists of the amino acid sequence of SEQ ID NO:

284.

81. The ISVD according to any of one claims 1 to 80, which amino acid sequence i) has 80% amino acid sequence identity with one of the amino acid sequences of SEQ ID NOs: 4, 5, 7, 311-525 or 527-528, in which for the purposes of determining the degree of amino acid identity, the amino acid residues that form the CDR sequences are disregarded; and in which: ii) preferably one or more of the amino acid residues at positions 11, 37, 44, 45, 47, 83, 84, 103, 104 and 108 according to the Kabat numbering are chosen from the Hallmark residues mentioned in Table 1.

82. The ISVD according to any one of claims 1 to 81, that essentially consists of a heavy chain variable domain sequence that is derived from a conventional four-chain antibody or that essentially consist of a heavy chain variable domain sequence that is derived from heavy chain antibody.

83. The ISVD according to any one of claims 1 to 82, that essentially consists of a VHH, a humanized VHH, a camelized VH, a domain antibody, a single domain antibody, or a dAb.

84. The ISVD according to any one of claims 1 to 83, which is a humanized ISVD that is chosen from the group consisting of SEQ ID NOs: 5, 7, 311-321, 326-336, 341-378, 380-418, 421-449, 451- 481, 483-496, 498-512 and 527-528, or from the group consisting of amino acid sequences thathave more than 80%, preferably more than 90%, more preferably more than 95%, such as 99% or more amino acid sequence identity with at least one of the amino acid sequences of SEQ ID NOs: 5, 7, 311-321, 326-336, 341-378, 380-418, 421-449, 451-481, 483-496, 498-512 and 527- 528.

85. The ISVD according to any one of claims 1 to 84 in which the amino acid sequence is chosen from the group consisting of SEQ ID NO: 4, 5, 7, 311-525, and 527-528.

86. A polypeptide or construct that comprises or essentially consists of one or more ISVDs according to any one of claims 1 to 85, and optionally further comprises one or more other groups, residues, moieties or binding units, optionally linked via one or more linkers.

87. The polypeptide or construct according to claim 86, in which said one or more other groups, residues, moieties or binding units are amino acid sequences.

88. The polypeptide or construct according to any one of claims 86 to 87, in which said one or more linkers are one or more amino acid sequences.

89. The polypeptide or construct according to any one of claims 86 to 88, in which said one or more other groups, residues, moieties or binding units are immunoglobulin sequences.

90. The polypeptide or construct according to any one of claims 86 to 89, in which said one or more other groups, residues, moieties or binding units are ISVDs.

91. The polypeptide or construct according to any one of claims 86 to 90, in which said one or more other groups, residues, moieties or binding units are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies and dAbs.

92. The polypeptide or construct according to any one of claims 86 to 91, which is a multivalent construct.

93. The polypeptide or construct according to any one of claims 86 to 92, which is a multispecific construct.

94. The polypeptide or construct according to any one of claims 86 to 93, in which said one or more other groups, residues, moieties or binding units provide the polypeptide or construct with increased half-life, compared to the ISVD without the one or more other groups, residues, moieties or binding units.

95. The polypeptide or construct according to claim 94, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of a polyethylene glycol molecule (PEG), serum proteins or fragments thereof, binding units that specifically bind to serum proteins, an Fc portion, and small proteins or peptides that specifically bind to serum proteins.

96. The polypeptide or construct according to claim 95, in which said one or more other groups, residues, moieties or binding units that provide the polypeptide or construct with increased half-life is chosen from the group consisting of human serum albumin or fragments thereof.

97. The polypeptide or construct according to claim 95, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of binding units that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG).

98. The polypeptide or construct according to claim 97, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life are chosen from the group consisting of VHHs, humanized VHHs, camelized VHs, domain antibodies, single domain antibodies, or dAbs that specifically bind to serum albumin (such as human serum albumin) or a serum immunoglobulin (such as IgG).

99. The polypeptide or construct according to claim 98, in which said one or more other groups, residues, moieties or binding units that provides the polypeptide or construct with increased half-life is an ISVD that specifically binds human serum albumin.

100. The polypeptide or construct according to claim 99, wherein said ISVD that specifically binds human serum albumin essentially consists of 4 framework regions (FR1 to FR4, respectively) and 3 complementarity determining regions (CDR1 to CDR3 respectively), in which: - CDR1 (AbM numbering) has an amino acid sequence selected from: a) the amino acid sequence of SEQ ID NO: 43; b) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 43; and c) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequences of SEQ ID NO: 43; and - CDR2 (AbM numbering) has an amino acid sequence selected from: d) the amino acid sequence of SEQ ID NO: 45; e) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 45; f) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO: 45; and - CDR3 (AbM numbering) has an amino acid sequence selected from: g) the amino acid sequence of SEQ ID NOs: 47; h) amino acid sequences that have at least 80% amino acid identity with the amino acid sequence of SEQ ID NO: 47; i) amino acid sequences that have 3, 2, or 1 amino acid difference with the amino acid sequence of SEQ ID NO:

47.

101. The polypeptide or construct according to claim 100, wherein CDR1 consists of the amino acid sequence of SEQ ID NO: 43, CDR2 consists of the amino acid sequence of SEQ ID NO: 45, and CDR3 consists of the amino acid sequence of SEQ ID NO:

47.

102. The polypeptide or construct according to claim 101, wherein said ISVD that specifically binds human serum albumin is selected from the group consisting of ALB8 (SEQ ID NO: 27), ALB23 (SEQ ID NO: 28), ALBX00001 (SEQ ID NO: 41) and ALB23002 (SEQ ID NO: 42).

103. The polypeptide or construct according to anyone of claims 86 to 102, wherein said linker is chosen from the group consisting of SEQ ID NOs: 53 to 69.

104. The polypeptide or construct according to any of claims 86 to 103, further comprising a C- terminal extension.

105. The polypeptide or construct according to claim 104, wherein said C-terminal extension is a C- terminal extension (X)n, in which n is 1 to 10, preferably 1 to 5, such as 1, 2, 3, 4 or 5 (and preferably 1 or 2, such as 1); and each X is an (preferably naturally occurring) amino acid residue that is independently chosen, and preferably independently chosen from the group consisting of alanine (A), glycine (G), valine (V), leucine (L) or isoleucine (I).

106. A nucleic acid that encodes an ISVD according to any one of claims 1 to 85, or a polypeptide according to any one of claims 86 to 105.

107. The nucleic acid according to claim 106, that is in the form of a genetic construct.

108. A non-human host or host cell that expresses, or that under suitable circumstances is capable of expressing, an ISVD according to any of one claims 1 to 85, or a polypeptide according to any one of claims 86 to 105; and / or that comprises the nucleic acid according to any one of claims 106 or 107.

109. A method for producing an ISVD according to any one of claims 1 to 85, or a polypeptide according to any one of claims 86 to 105, at least comprising the steps of: a) expressing, in a suitable non-human host cell or host organism or in another suitable expression system, a nucleic acid according to any one of claims 106 or 107; optionally followed by: b) isolating and / or purifying the ISVD according to any one of claims 1 to 85, or the polypeptide according to any one of claims 86 to 105.

110. A method for producing an ISVD according to any one of claims 1 to 85, or a polypeptide according to any one of claims 86 to 105, said method at least comprising the steps of:a) cultivating and / or maintaining a non-human host or host cell according to claim 108 under conditions that are such that said non-human host or host cell expresses and / or produces at least one ISVD according to any one of claims 1 to 85, or at least one polypeptide according to any one of claims 86 to 105; optionally followed by: b) isolating and / or purifying the ISVD according to any one of claims 1 to 85, or polypeptide according to any one of claims 86 to 105.

111. A composition comprising at least one ISVD according to any one of claims 1 to 85, at least one polypeptide or construct according to any one of claims 86 to 105, or at least one nucleic acid according to any one of claims 106 or 107.

112. The composition according to claim 111, which is a pharmaceutical composition.

113. The composition according to any one of claims 111 or 112, which is a pharmaceutical composition, that further comprises at least one pharmaceutically acceptable carrier, diluent or excipient and / or adjuvant, and that optionally comprises one or more further pharmaceutically active polypeptides and / or compounds.

114. The ISVD according to any one of claims 1 to 85, the polypeptide or construct according to any one of claims 86 to 105, or the composition according to any one of claims 111 to 113, for use as a medicament.

115. The ISVD according to any one of claims 1 to 85, the polypeptide or construct according to any one of claims 86 to 105, or the composition according to any one of claims 111 to 113, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder.

116. The ISVD according to any one of claims 1 to 85, the polypeptide or construct according to any one of claims 86 to 105, or the composition according to any one of claims 111 to 113, for use in the diagnosis, prevention and / or treatment of at least one disease and / or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved.

117. The ISVD according to any one of claims 1 to 85, the polypeptide or construct according to any one of claims 86 to 105, or the composition according to any one of claims 111 to 113, for use in the diagnosis, prevention and / or treatment of cancer.

118. A method for the diagnosis, prevention and / or treatment of at least one disease and / or disorder, comprising the administration, to a subject in need thereof, of a pharmaceutically active amount of an ISVD according to any one of claims 1 to 85, a polypeptide or construct according to any one of claims 86 to 105, or a composition according to any one of claims 111 to 113.

119. The method according to claim 118, for the diagnosis, prevention and / or treatment of at least one disease or disorder that is associated with PD-L1, with its biological or pharmacological activity, and / or with the biological pathways or signalling in which PD-L1 is involved, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of claims 1 to 85, a polypeptide or construct according to any one of claims 86 to 105, or composition according to any one of claims 111 to 113.

120. The method according to any one of claims 118 or 119, for the diagnosis, prevention and / or treatment of cancer, said method comprising administering, to a subject in need thereof, a pharmaceutically active amount of at least one ISVD according to any one of claims 1 to 85, a polypeptide or construct according to any one of claims 86 to 105, or a composition according to any one of claims 111 to 113.

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