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.
|CR #||Name||Molecular weight (Da, approx.)||Ligand||CD||Major cell typesa||Major activities|
|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|
- 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.