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 gamma Rs: RI (CD64), RII (CD32), and RIII (CD16), which generate multiple isoforms, are recognized. These receptors function in the activation or inhibition of immune responses. The activating-type receptor either has, or associates non-covalently with an accessory subunit (FcR gamma or zeta  chain) that has an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain. In contrast, the inhibitory receptor (Fc gamma RIIB) has a built-in immunoreceptor tyrosine-based inhibitory motif (ITIM) in its own cytoplasmic domain. Fc gamma RI is a high-affinity receptor that binds monomeric IgG. Both Fc gamma RII and RIII are low-affinity receptors that bind IgG in the form of immune complexes. Two genes for human Fc gamma RIII, A and B, encoding a transmembrane receptor and a glycosylphosphatidylinositol (GPI) anchored protein, respectively, have been identified. Three allelic variants of Fc gamma RIIIB, NA-1, NA-2, and SH, exist. A soluble form of Fc gamma RIIIB corresponding to the extracellular region of the receptor is produced by proteolytic cleavage and circulates in plasma and other body fluids. The extracellular domains of Fc gamma RIIIA and B share 97% amino acid sequence homology.

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