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]
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.) | Major cell types | Major activities | |||
|---|---|---|---|---|---|---|---|
| CR1 | 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 | 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 | 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 | D, Mac, M0, PMN | Leukocyte adhesion | ||
| C3AR1 | 75,000 | C3a | – | Endo, MC, Pha | Cell activation | ||
| C5AR1 | 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.
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]