FcγRIII, synonymously also referred to CD16, is a type III Fcγ receptor which has other two identified members: a high-affinity receptor FcγRI (CD64) and a low-affinity one FcγRII (CD32). This transmembrane protein, FcγRIII, contains two extracellular immunoglobulin (Ig)-like domains with low-affinity for IgG. It is responsible for antibody-dependent cellular cytotoxicity. FcγRIII is expressed on large granular lymphocytes of both nature killer (NK)- and T-cell types. There are two different isoforms of it in humans named FcγRIIIa and FcγRIIIb, sharing 96% sequence identity in extracellular Ig binding regions. FcγRIIIa is a transmembrane receptor found on macrophages, mast and NK cells. FcγRIIIb is expressed exclusively on neutrophils and functions as the only Fc receptor anchored via a glycosyl-phosphatidylinositol (GPI) linker to the cell membrane. FcγRIIB (Fc fragment of IgG receptor IIb), aslo known as Fc gamma RIIB, FCGR2B or CD32B, is a 34 kDa protein that is encoded by the FCGR2B gene located on the chromosome 1q23.3. It has a length of 310 amino acids and produces a low-affinity inhibitory receptor for immunoglobulin G (IgG). In humans, FcγRIIB is composed of two alternatively spliced subunits, FcγRIIb1 and FcγRIIb2. It is a predominant Fc receptor (FcR) on B cells and is a founding member of immunoreceptor tyrosine-based inhibitory motif (ITIM) family of immune inhibitory receptors. A motif within the cytoplasmic tail of FcγRIIb termed ITIM, confers the ability to inhibit cellular activation mediated by immunoreceptor tyrosine-based inhibition motif (ITAM)-bearing receptors. Inhibition only occurs when FcγRIIB is co-clustered with an ITAM-bearing immunoreceptor.