FcRIIIb

Fc gamma receptor IIIb (FcγRIIIb), commonly known as CD16b, is a low-affinity receptor for the Fc portion of IgG antibodies. It is a member of the Fc gamma receptor family and is encoded by the FCGR3B gene on chromosome 1. FcγRIIIb is primarily expressed on neutrophils and, to a lesser extent, on eosinophils and some other leukocytes. Unlike FcγRIIIA/CD16a, FcγRIIIb is tethered to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor and lacks a transmembrane and cytoplasmic signaling domain, making signaling dependent on association with other receptors or membrane components.

Functionally, FcγRIIIb binds immune complexes and facilitates neutrophil recognition and uptake of opsonized particles. It contributes to phagocytosis, clearance of immune complexes, and regulation of neutrophil activation. Because it is a low-affinity receptor, it preferentially binds multivalent IgG immune complexes rather than monomeric IgG. The receptor can also participate in signaling indirectly through cross-linking with other receptors on the neutrophil surface.

FcγRIIIb exhibits copy number variation and allelic polymorphisms, including the NA1 and NA2 allotypes, which influence receptor density and possibly affinity for IgG subclasses. Variation in FCGR3B copy number or allotype has been associated in some studies with altered susceptibility to autoimmune diseases (notably systemic lupus erythematosus) and infectious diseases, as well as with changes in neutrophil functions such as phagocytosis and immune complex handling. The receptor can be shed from the membrane by phospholipases, yielding soluble FcγRIIIb in plasma under certain conditions.

FcγRIIIb is distinct from FcγRIIIA (CD16a), which is a transmembrane receptor on natural killer cells and macrophages capable of signaling and mediating antibody-dependent cellular cytotoxicity.