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.<ref name="ar14">,

 Complement and its receptors: new insights into human disease, 
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 DOI: 10.1146/annurev-immunol-032713-120154,
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 DOI: 10.1016/j.vaccine.2008.11.022,
 PMID: 19388161,
 PMC: 4018718,</ref> Different complement receptors can participate in either the classical complement pathway, the alternative complement pathway, or both.<ref name="Janeway 2001">, 
  
 Immunobiology: The Immune System in Health and Disease. online version, 
 5th edition, 
 New York:Garland Science, 
 2001, 
  
  
  
  
  
 Accessed: 2020-06-17.</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>Peter,

 The Immune System, 
 2nd edition, 
 Garland Science, 
  
  
  
 ISBN 9780815340935,</ref>

CR #	Name	Molecular weight (Da, approx.)<ref name="ar14"/>	Ligand<ref name="Janeway 2001" />	CD	Major cell types<ref name="Janeway 2001"/>	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 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.

Clinical significance

Main articles: Complement system § Role in disease, Classical complement pathway § Clinical significance, and Alternative complement pathway § Role in disease

Deficits in complement receptor expression can cause disease.<ref name="eMedicine Dermatology">

Complement Receptor Deficiency: eMedicine Dermatology(link). Medscape.

Accessed 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

External links

• Complement+receptors at the US National Library of Medicine Medical Subject Headings (MeSH)

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