CD16, also known as low affinity IgG receptor III (FcgammaRIII), is expressed as two distinct forms, referred to as CD16a and CD16b. CD16a (FcgammaRIIIA) is a 50-65 kD polypeptide-anchored transmembrane protein expressed on the surface of NK cells, activated monocytes, macrophages, a subset of T cells, and placental trophoblasts in humans. CD16b (FcgammaRIIIB) is a 48 kD glycosylphosphatidylinositol (GPI)-anchored protein whose extracellular domain is over 95% homologous to that of CD16a, and is expressed specifically on neutrophils. CD16 binds to aggregated IgG or IgG-antigen complex, which functions in NK cell activation, phagocytosis, and antibody-dependent cell-mediated cytotoxicity (ADCC).
