The binding of rabbit IgG and its enzymatically derived fragments to homologous peritoneal macrophages

Rabbit IgG and its Fab, Fc and pFc' fragments, prepared by papain or peptic digestion, were assayed for binding to homologous peritoneal macrophages. The binding affinity of IgG for the peritoneal macrophages (Ka = 5.9 +/- 1.6 x 10(5) L/M) was comparable to that recorded with alveolar macrophages (7.6 +/- 1.8 x 10(5) L/M, Arend & Mannik, 1973) but the number of receptor sites per peritoneal cell (4.6 +/- 2.1 x10(6)) was about four-fold greater than on the latter. Of the fragments, only Fc bound to macrophages with an affinity comparable to intact IgG; pFc' bound weakly and Fab was totally inactive. These data, taken with a recent study involving rabbit IgG and guinea-pig macrophages (Ovary, Saluk, Quijada & Lamm, 1976), indicate that the primary IgG binding site for macrophages is located in the C gamma 2 domain.