Complement receptor

{{Use dmy dates|date=March 2024}} {{Pfam_box | Symbol = Complement receptor | Name = Complement receptor | image = | width = | caption = | InterPro= | SMART= | PROSITE = | SCOP = | TCDB = | OPM family= | OPM protein= | Pfam= | PDB= | Membranome family= 116 }} 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.<ref name="ar14">{{cite journal | vauthors = Holers VM | title = Complement and its receptors: new insights into human disease | journal = Annual Review of Immunology | volume = 32 | pages = 433–59 | date = 29 January 2014 | pmid = 24499275 | doi = 10.1146/annurev-immunol-032713-120154 | doi-access = free }}</ref> Complement receptor activation contributes to the regulation of inflammation, leukocyte extravasation, and phagocytosis; it also contributes to the adaptive immune response.<ref name="cr17">{{cite book |vauthors=Verschoor A, Kemper C, Köhl J |title = Encyclopedia of Life Sciences |chapter = Complement Receptors |date = 15 September 2017 |pages=1–17 |doi=10.1002/9780470015902.a0000512.pub3 |isbn=9780470015902 }}</ref><ref name="pmid19388161">{{cite journal | vauthors = Carroll MC | title = Complement and humoral immunity | journal = Vaccine | volume = 26 | issue = Suppl 8 | pages = I28-33 | date = December 2008 | pmid = 19388161 | pmc = 4018718 | doi = 10.1016/j.vaccine.2008.11.022 }}</ref> Different complement receptors can participate in either the classical complement pathway, the alternative complement pathway, or both.<ref name="Janeway 2001">{{cite book|url=https://www.ncbi.nlm.nih.gov/books/NBK27100/|chapter=The complement system and innate immunity| vauthors = Janeway Jr CA, Travers P, Walport M, Shlomchik MJ |year=2001|edition=5th|title=Immunobiology: The Immune System in Health and Disease|publisher=Garland Science|access-date=2020-06-17|location=New York }}</ref>

== 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.<ref name="ar14"/> 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.<ref>{{cite book | first = Peter | last = Parham | name-list-style = vanc |title=The Immune System | year = 2005 |edition=2nd|publisher=Garland Science|isbn=9780815340935}}</ref>

{| class="wikitable" ! CR # || Name || Molecular weight (Da, approx.)<ref name="ar14"/> || Ligand<ref name="Janeway 2001" /> || CD || Major cell types<ref name="Janeway 2001"/>{{ref|A|a}} || Major activities<ref name="ar14"/> |- | 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 α<sub>M</sub>β<sub>2</sub>" || 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.{{note|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 | Complement system#Role in disease | Classical complement pathway#Clinical significance | Alternative complement pathway#Role in disease}} Deficits in complement receptor expression can cause disease.<ref name="eMedicine Dermatology">{{cite web | vauthors = Schwartz RA, Thomas I |url=http://emedicine.medscape.com/article/1051238-overview |title=Complement Receptor Deficiency: eMedicine Dermatology |website=Medscape |access-date=2010-12-07}}</ref> Mutations in complement receptors which alter receptor function can also increase risk of certain diseases.<ref name="ar14"/>

== See also == * Complement system * Humoral immunity * Immune system

== References == {{Reflist}}

== External links == * {{MeshName|Complement+receptors}}

Category:Complement system Category:Single-pass transmembrane proteins