CD16 (FcgRIII) is a 50-65 kDa glycoprotein serving as a low affinity IgG receptor. Human FcgRIII is expressed in two forms FcgRIII-A and -B. FcgRIII-A is a transmembrane protein of monocytes, macrophages, NK cells and a subset of T cells. It is associated with FceRI-g subunit and is responsible for antibody-dependent NK cell cytotoxicity. Mast cell FcgRIII-A is associated, moreover, with FceRI-b subunit. Besides IgG, FceRI-A can be triggered also by oligomeric IgE. FcgRIII-B is a GPI-linked monomeric receptor expressed on neutrophils and is involved in their activation and induction of a proadhesive phenotype.