Fc gamma RIIIA/CD16a is a low/intermediate affinity activating receptor for polyvalent immune-complexed IgG. It is involved in phagocytosis, secretion of enzymes and inflammatory mediators, antibody-dependent cytotoxicity, and clearance of immune complexes. It is expressed on NK cells, T cells, monocytes, and macrophages. It is closely related to the GPI-linked Fc gamma RIIIB which is expressed on neutrophils and eosinophils. Surface expression requires interaction with an accessory chain, either the common gamma-chain or CD3 zeta. Shed forms of both Fc gamma RIIIA and Fc gamma RIIIB can be generated by proteolytic cleavage and retain binding activity. Soluble Fc gamma RIII circulates in normal plasma and is elevated in rheumatoid arthritis and in coronary artery diseases.