Receptors for the Fc region of IgG (Fc gamma R) are members of the Ig superfamily. Based on their genetic organization and molecular structure, three classes of human Fc γ Rs can be identified: RI (CD64), RII (CD32), and RIII (CD16), which produce a variety of isoforms. Fc γ RI is a high-affinity receptor that binds monomeric IgG. Both Fc γ RII and RIII are low-affinity receptors that bind IgG as immune complexes. Two genes have been identified for human Fc γ RIII, A and B, which encode a transmembrane receptor and a glycosylphosphatidylinositol (GPI) anchored protein, respectively. Three allelic variants of Fc γ RIIIB, NA-1, NA-2, and SH were present. CD16 inhibits a variety of cellular functions such as B cell activation/proliferation and mast cell degranulation, cannot mediate antibody-dependent cytotoxicity and phagocytosis, acts as a trap for peripheral circulating immune complexes, and binds IgG complexes but does not activate neutrophils. CD16 regulates the inflammatory process by interacting with CR3 and CR4 and inducing IL-6 and IL-8 production by monocytes.
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