FcγRIIb-CD32b, is the sole inhibitory receptor among the classical Fc gamma receptors for IgG.
FcγRIIB (also known as CD32B) is the sole inhibitory Fc gamma receptor that regulates immune responses by binding the Fc region of IgG antibodies.
Low-affinity receptor for the Fc portion of complexed or aggregated IgG (not monomeric IgG).
Structure: Type I transmembrane glycoprotein with two extracellular Ig-like domains, a transmembrane region, and a cytoplasmic tail containing an immunoreceptor tyrosine-based inhibitory motif (ITIM).
Cellular Distribution: It is expressed on B cells, dendritic cells, macrophages, mast cells, and neutrophils.
It is notably the only Fcγ receptor found on B cells.
Isoforms: Two main splice variants in humans: FcγRIIb1 — primarily on B cells; contains an insert that limits endocytosis. FcγRIIb2 — on myeloid cells; more readily internalized.
Broadly expressed on B cells (the only FcγR on B cells), monocytes/macrophages, dendritic cells, neutrophils, mast cells, basophils, and some other cell types.
Not typically found on NK cells.
When co-engaged (usually by immune complexes) with activating receptors such as the B-cell receptor (BCR) or activating FcγRs (FcγRI, FcγRIIa, FcγRIIIa), the ITIM is phosphorylated.
This recruits the phosphatases and raises the activation threshold of B cells and suppress antibody production.
Inhibits phagocytosis, pro-inflammatory cytokines released, and other effector functions of myeloid cells.
Regulates antigen presentation and dendritic-cell maturation.
It can also compete for immune-complex binding independently of the ITIM) tyrosine-based inhibitory motif.
Biological and clinical relevance
Critical for maintaining immune homeostasis and B-cell tolerance.
Polymorphisms that reduce FcγRIIb expression or function are associated with increased risk of systemic lupus erythematosus (SLE) and other autoimmune diseases.
A loss-of-function variant that raises autoimmunity risk may have been selected because it protects against severe malaria.
Emerging roles include modulation of intravenous immunoglobulin (IVIg) effects and potential involvement in neurodegenerative processes such as alpha-synuclein handling.
Therapeutic target: antibodies that engage or block FcγRIIb are in clinical development for autoimmune diseases and certain B-cell malignancies.
FcγRIIb acts as a key brake on IgG-driven immune responses.
Inhibitory Signaling: It contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic domain that recruits phosphatases like SHIP1/2 to suppress activating signals.
It raises the threshold for B cell activation, prevents autoimmunity, and dampens inflammation and phagocytosis.
Clinical Relevance: Genetic variations or dysregulation of FcγRIIB are linked to autoimmune conditions like systemic lupus erythematosus, and the receptor can influence the efficacy of monoclonal antibody therapies.