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.
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 # Name Molecular weight (Da, approx.) Ligand CD Major cell types Major activities CR1 Complement receptor 1 190,000–250,000C3b, 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 2 145,000C3d, 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 chainiC3b 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 chainiC3b CD11c+CD18 D, Mac, M0, PMN Leukocyte adhesion C3AR1 C3a receptor 75,000C3a – Endo, MC, Pha Cell activation C5AR1 C5a receptor 50,000 C5a CD88 Endo, MC, Pha Cell activation, immune polarization, chemotaxisC5AR2C5a receptor 236,000C5a –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
Deficits in complement receptor expression can cause disease. Mutations in complement receptors which alter receptor function can also increase risk of certain diseases.
See also Complement system Humoral immunity Immune system
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