Induction of an Fc conformational change by binding of antigen: the generation of protein A-reactive sites in chicken immunoglobulin.

Chicken antibody alone does not bind to Staphylococcus aureus or protein A-Sepharose. Immune complexes containing chicken antibodies do bind, however, showing protein A-binding sites exist on the molecules. Binding does not occur via the antigen contained in the complex. Complexes formed in far antigen excess also bind, suggesting that an induced conformational change in the Fc region of the immunoglobulin occurs but only a fraction of the total chicken antibody is reactive.