Receptors for the Fc region of IgG (Fc gamma R) are members of the Ig superfamily that function in the activation or inhibition of immune responses. Three classes of human Fc gamma Rs: RI (CD64), RII (CD32), and RIII (CD16), which generate multiple isoforms, are recognized. 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 aggregated or immune complexed IgG (IC).The extracellular domain of human Fc gamma RIIA shares approximately 90% amino acid sequence homology with human Fc gamma RIIB and Fc gamma RIIC. Fc gamma RIIA is expressed on many immune cell types (macrophage, neutrophil, eosinophils, platelets, dendritic cells and Langerhan cells) where inhibitory ITIM-bearing receptors may also be coexpressed and co-engaged by specific ligands.

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