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.

Expression and function

s, 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 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 also express CR1, which enables RBCs to carry complement-bound antigen-antibody complexes to the liver and spleen for degradation.
CR #NameMolecular weight LigandCDMajor cell typesMajor activities
CR1Complement receptor 1190,000–250,000C3b, C4b, iC3bCD35B, E, FDC, Mac, M0, PMNImmune complex transport ; phagocytosis ; immune adhesion ; cofactor and decay-acceleration; secondary Epstein-Barr virus receptor
CR2Complement receptor 2145,000C3d, iC3b, C3dg, Epstein-Barr virusCD21B, FDCB cell coactivator, primary Epstein-Barr virus receptor, CD23 receptor
CR3Macrophage-1 antigen or "integrin αMβ2"170,000 α chain + common 95,000 β chainiC3bCD11b+CD18FDC, Mac, M0, PMNLeukocyte adherence, phagocytosis of iC3b-bound particles
CR4Integrin alphaXbeta2 or "p150,95"150,000 α chain + common 95,000 β chainiC3bCD11c+CD18D, Mac, M0, PMNLeukocyte adhesion
C3AR1C3a receptor75,000C3a-Endo, MC, PhaCell activation
C5AR1C5a receptor50,000C5aCD88Endo, MC, PhaCell activation, immune polarization, chemotaxis

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.