The invention relates to a method for manufacturing a recombinant polyclonal
protein composition, in particular a recombinant polyclonal
antibody composition. The method comprises obtaining a collection of cells transfected with a
library of variant
nucleic acid sequences, wherein each call in the collection is transfected with and capable of expressing one member of the
library, which encodes a distinct member of a polyclonal
protein that binds a particular
antigen and which is located at the same
single site in the
genome of individual cells in said collection, wherein said
nucleic acid sequence is not naturally associated with said
cell in the collection. The cells are cultured under suitable conditions for expression of the polyclonal
protein, which is obtained from the cells or culture supernatant. The
nucleic acid sequence id introduced into the cells by
transfection with a
library of vectors for site-specific integration. The
present method is suitable for manufacturing recombinant
polyclonal antibodies, thereby making available a superior replacement of
plasma-derived therapeutic immunoglobulin products.