HUMAN MONOCYTES - DISTINCT RECEPTOR SITES FOR THIRD COMPONENT OF COMPLEMENT AND FOR IMMUNOGLOBULIN G

Human monocytes contain two distinict receptor sites, one specific for the third component of complement (C′3), the other for immunoglobulin G (γG). The two receptors may function either independently or cooperatively it the induction of phagocytosis. Ingestion of erythrocytes coated with immunoglobullin M antibody requires a relatively large number of bound C′3 molecules per cell. Ingestion of erythrocytes sensitized with γG antibody is independent of complement; however, the reaction is inhibited by concentrations of γG far below those in normal serum. Inhibition by γG-globulin is overcome by a relatively small number of bound C′3 molecules per cell. The two monocyte receptors exert a cooperative effect on ingestion by monocytes of erythrocytes coated with γG antibody in the presence of inhibitory amounts of free γG.