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.^[1] Complement receptor activation contributes to the regulation of inflammation, leukocyte extravasation, and phagocytosis; it also contributes to the adaptive immune response.^[2][3] Different complement receptors can participate in either the classical complement pathway, the alternative complement pathway, or both.^[4]

Complement receptor
Identifiers
Symbol	Complement receptor
Membranome	116

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.^[1] 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.^[5]

CR #	Name	Molecular weight (Da, approx.)[1]	Ligand[4]	CD	Major cell types[4]a	Major activities[1]
CR1	Complement receptor 1	190,000–250,000	C3b, C4b, iC3b	CD35	B, E, FDC, Mac, M0, PMN	Immune complex transport (E); phagocytosis (PMN, Mac); immune adhesion (E); cofactor and decay-acceleration; secondary Epstein-Barr virus receptor
CR2	Complement receptor 2	145,000	C3d, iC3b, C3dg, Epstein-Barr virus	CD21	B, FDC	B cell coactivator, primary Epstein-Barr virus receptor, CD23 receptor
CR3	Macrophage-1 antigen or "integrin αMβ2"	170,000 α chain + common 95,000 β chain	iC3b	CD11b+CD18	FDC, Mac, M0, PMN	Leukocyte adherence, phagocytosis of iC3b-bound particles
CR4	Integrin alphaXbeta2 or "p150,95"	150,000 α chain + common 95,000 β chain	iC3b	CD11c+CD18	D, Mac, M0, PMN	Leukocyte adhesion
C3AR1	C3a receptor	75,000	C3a	–	Endo, MC, Pha	Cell activation
C5AR1	C5a receptor	50,000	C5a	CD88	Endo, 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.^[6] Mutations in complement receptors which alter receptor function can also increase risk of certain diseases.^[1]

See also

• Complement system
• Humoral immunity
• Immune system