Phagocytosis of IgG-opsonized MRBCs. Thioglycolate-elicited peritoneal macrophages from FcγRIII wild-type (□) and FcγRIII KO (▪) mice were incubated with MRBCs opsonized with (A) pathogenic MRBC MoAbs of the IgG1 (105-2H) and IgG2a (34-3C) isotypes or with medium alone. In addition, macrophages from FcγRIII wild-type (B) and FcγRIII KO (C) mice were first incubated with (▪) or without (□) the 2.4G2 antibody, which is directed against FcγRII and FcγRIII, and subsequently opsonized with the MRBC 105-2H and 34-3C. After 1 hour of incubation at 37°C, extracellular erythrocytes were lysed by hypotonic shock and the percentage of positive peritoneal macrophages that had ingested more than two erythrocytes was assessed microscopically. Results are expressed as the mean values ± SEM of five individual experiments. Significances are determined by Student’s t-test (*P< .05; **P < .001).