Phagocytosis of immune complexes by human neutrophils and monocytes: relative importance of Fc and C3b receptors.

Human neutrophils and peripheral blood monocytes were studied for their Fc- and C3b-receptors after incubation of the cells with two types of immune complexes (IgG or F(ab')2. Immune complexes can bind to and activate phagocytes via Fc- and C3b-receptors. On mononuclear phagocytes an additional receptor is found that is able to interact directly with F(ab')2-complexes. C3b- and Fc-receptors can mediate ingestion of appropriately opsonized particles, C3b-receptors being at least as efficient as Fc-receptors. The extents of cell activation and phagocytosis in vitro depend on a number of variables, like complex size, ligand (antibody, C3b) density on the surface of opsonized particles, receptor number etc., which are not controlled in most studies and thus may explain the discrepant results reported in the literature. With the in vitro results obtained here, we cannot fully explain in vivo data obtained by various groups, where F(ab')2 antibodies yielded full protection against different types of infections. Therefore, great caution must be applied in all attempts to extrapolate in vitro data to the actual situation in vivo.