Receptors for the Fc region of IgG (Fc γ R) are members of the Ig superfamily that function in the activation or inhibition of immune responses. Human Fc gamma Rs are classified into three classes: RI (CD64), RII (CD32), and RIII (CD16), which give rise to multiple isoforms (1-3). Fc gamma RI is a high-affinity receptor that binds monomeric IgG, and Fc gamma RII and RIII are low-affinity receptors that bind aggregated or immune complex IgG (IC). The extracellular domain of human Fc gamma RIIA shares approximately 90% amino acid sequence identity with human Fc gamma RIIB and Fc gamma RIIC. Fc gamma RIIA is expressed on many immune cell types: macrophages, neutrophils, eosinophils, platelets, dendritic cells, and Langerhans cells, where inhibitory ITIM receptors may also be co-expressed and co-engaged by specific ligands. Signaling through FcγRIIA leads to the initiation of inflammatory responses (cytolysis, phagocytosis, degranulation, and cytokine production) that can be modulated by signals from inhibitory receptors. The strength of the signal depends on the ratio of activated and inhibitory receptor expression.