Human Monocytes: Distinct Receptor Sites for the Third Component of Complement and for Immunoglobulin G

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Abstract
Human monocytes contain two distinct 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 in the induction of phagocytosis. Ingestion of erythrocytes coated with immunoglobulin 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.