Human IgG Fc receptors.

Considerable recent progress has been made in our understanding of how IgG immune complexes interact with plasma membrane Fc receptors (FcR) to mediate a diversity of biological responses in man. These responses include endocytosis of antibody complexes; stimulation of the secretion of various inflammatory mediators such as enzymes, oxygen products, and arachidonic acid derivatives; antibody-mediated cell killing by FcR-bearing cells; immune complex-mediated stimulation of the platelet release reaction; modulation of the immune response; and others. The Fc receptors responsible for transducing these responses have largely been characterized. They fall into three distinct classes (FcRI, FcRII, FcRIII) based upon a number of criteria including molecular size, affinity, and specificity for ligand, cell-specific display, and epitope expression; but with a few possible exceptions these three classes do not seem to correlate with the multiple functional properties of the receptors. Recently the primary structures of these molecules have been inferred from cDNA sequences. As members of the Ig gene superfamily, they all have extracellular portions consisting of two or three truncated disulfide-looped Ig-like domains. As a rule, these are rather conventional integral membrane glycoproteins bearing a short lipid-spanning polypeptide and a cytoplasmic tail of varying length, although one appears to be linked to the plasma membrane by a glycosyl phosphatidylinositol moiety and thus bears no cytoplasmic domain. Remarkably, within each class of FcR there appears to be considerable polymorphism, especially within the cytoplasmic portions. How these various structures dictate specific biological consequences is currently under study.