Binding of immune complexes of guinea pig IgG2 to homologous peritoneal exudate cells

A quantitative study has been performed to correlate the macrophage‐binding activity of immune complexes of guinea pig IgG2 with the antigen‐antibody composition and size of the immune complex. Binding, measured as (a) uptake at a single IgG concentration, (b) resistance to IgG2 monomer inhibition and (c) mean binding avidity of the complex, was maximum at the antigen‐antibody ratio optimum for immune precipitation. All three parameters declined in value with increasing proportions of antigen in the complex. Gel filtration of immune complexes revealed that complex size and binding avidity declined with increase in antigen excess. The relationship of binding activity to complex size and the similar inhibitory patterns of IgG2 and its fragments for binding of monomer and complexed IgG2 suggest that binding enhancement results from the cooperative action of multiple Fc sites in the immune oligomer, rather than from conformational change.