complement receptor complement receptor a complement receptor is a membrane-bound receptor belonging to the complement system, which is part of the innate immune system. complement receptors bind effector protein fragments that are produced in response to antigen-antibody complexes or damage-associated molecules. complement receptor activation contributes to the regulation of inflammation, leukocyte extravasation, and phagocytosis; it also contributes to the adaptive immune response. different complement receptors can participate in either the classical complement pathway, the alternative complement pathway, or both. complement receptoridentifierssymbolcomplement receptormembranome116 expression and function white blood cells, particularly monocytes and macrophages, express complement receptors on their surface. all four complement receptors can bind to fragments of complement component 3 or complement component 4 coated on pathogen surface, but the receptors trigger different downstream activities. complement receptor (cr) 1, 3, and 4 function as opsonins which stimulate phagocytosis, whereas cr2 is expressed only on b cells as a co-receptor. red blood cells (rbcs) also express cr1, which enables rbcs to carry complement-bound antigen-antibody complexes to the liver and spleen for degradation. cr #namemolecular weight (da, approx.)ligandcdmajor cell typesamajor activities cr1complement receptor 1190,000–250,000 c3b, c4b, ic3bcd35b, e, fdc, mac, m0, pmn immune complex transport (e); phagocytosis (pmn, mac); immune adhesion (e); cofactor and decay-acceleration; secondary epstein-barr virus receptor cr2complement receptor 2145,000 c3d, ic3b, c3dg, epstein-barr viruscd21b, fdc b cell coactivator, primary epstein-barr virus receptor, cd23 receptor cr3macrophage-1 antigen or "integrin αmβ2"170,000 α chain + common 95,000 β chain ic3bcd11b+cd18fdc, mac, m0, pmn leukocyte adherence, phagocytosis of ic3b-bound particles cr4integrin alphaxbeta2 or "p150,95"150,000 α chain + common 95,000 β chain ic3bcd11c+cd18d, mac, m0, pmn leukocyte adhesion c3ar1c3a receptor75,000 c3a-endo, mc, pha cell activation c5ar1c5a receptor50,000 c5acd88endo, mc, pha cell activation, immune polarization, chemotaxis c5ar2 c5a receptor 2 36,000 c5a - chemotaxis a.^ b: b cell. e: erythrocyte. endo: endothelial cell. d: dendritic cell. fdc: follicular dendritic cell. mac: macrophage. mc: mast cell. m0: monocyte. pha: phagocyte. pmn: polymorphonuclear leukocyte. clinical significance main articles: complement system § role in disease, classical complement pathway § clinical significance, and alternative complement pathway § role in disease deficits in complement receptor expression can cause disease. mutations in complement receptors which alter receptor function can also increase risk of certain diseases.