Properties of the Fc Receptor on Macrophages and Monocytes

Abstract

Macrophages and monocytes possess a surface receptor specific for the Fc region of certain subclasses of IgG. The binding site on IgG is localized within the Cγ3 homology regions of the heavy chains. The intrinsic affinity of the receptor for IgG ranges from 10⁶ to 10⁸ M⁻1 depending on species and subclass of IgG. The most definitive studies on mouse macrophages indicate that IgG2a rapidly associates and dissociates from the receptor. The overall reaction is exothermic; increasing temperature lowers the intrinsic affinity. The Fc receptor, in common with many other membrane components, may be capped by polyvalent ligands under permissive conditions and capping is inhibited by azide. Data on the chemistry of the receptor is both sparse and conflicting. Sensitivity of the receptor to proteolytic enzymes has been clearly demonstrated for mouse macrophages although rabbit and guineapig cells appeared to carry resistant receptors. IgG-binding may be inhibited if cells are treated with phospholipases and certain group-specific reagents. Evidence is reviewed indicating that the Fc receptors found on various cell types are different. The macrophage Fc receptor appears to play a role in mediating phagocytosis and in non-imnune cytotoxicity. Whether the receptor serves only to concentrate sensitized target cells at the cell surface or whether occupation of the receptors results in modulation of effector cell function remains to be determined.