Complement receptor: Difference between revisions

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{{Multiple issues| {{expert-subject|date=September 2008}} {{refimprove|date=September 2008}}}}
{{Short description|Receptors that bind complement proteins}}
{{Use dmy dates|date=July 2013}}
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A '''complement receptor''' is a membrane-bound [[receptor (biochemistry)|receptor]] belonging to the [[complement system]], which is part of the [[innate immune system]]. Complement receptors bind [[Effector (biology)|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 }}</ref> Complement receptor activation contributes to the regulation of [[inflammation]], [[leukocyte extravasation]], and [[phagocytosis]]; it also contributes to the [[adaptive immune system|adaptive immune response]].<ref name="cr17">{{cite journal|title=Complement Receptors |vauthors=Verschoor A, Kemper C, Köhl J  |journal=eLS |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 Suppl 8 | 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 ==
== Complement Receptors ==
[[White blood cell]]s, particularly [[monocyte]]s and [[macrophage]]s, [[gene expression|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 [[opsonin]]s which stimulate [[phagocytosis]], whereas CR2 is expressed only on [[B cell]]s as a [[co-receptor]].
Complement receptors are integral membrane proteins that interact with components of the [[complement system]], a crucial part of the [[immune system]]. These receptors play a significant role in the immune response by recognizing and binding to complement proteins, thereby facilitating various immune functions such as phagocytosis, inflammation, and clearance of immune complexes.


[[Red blood cell]]s (RBCs) also express CR1, which enables RBCs to carry complement-bound [[antigen-antibody complex]]es to the [[liver]] and [[spleen]] for degradation.<ref>{{cite book | first = Peter | last = Parham | name-list-format = vanc |title=The Immune System |edition=2nd|publisher=Garland Science|isbn=9780815340935}}</ref>
== Types of Complement Receptors ==
Complement receptors are classified based on the specific complement components they bind to. The major types include:


{| class="wikitable"
=== CR1 (Complement Receptor 1) ===
! CR # || Name || Molecular weight (Da, approx.)<ref name="ar14"/> || [[Ligand (biochemistry)|Ligand]]<ref name="Janeway 2001" /> || [[Cluster of differentiation|CD]] || Major cell types<ref name="Janeway 2001"/>{{ref|A|a}} || Major activities<ref name="ar14"/>
CR1, also known as CD35, is a receptor for the complement components C3b and C4b. It is expressed on the surface of [[erythrocytes]], [[leukocytes]], and [[kidney]] cells. CR1 plays a role in the clearance of immune complexes and the regulation of complement activation.
|-
| 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
|}
: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==
=== CR2 (Complement Receptor 2) ===
{{Main | Complement system#Role in disease | Classical complement pathway#Clinical significance | Alternative complement pathway#Role in disease}}
CR2, or CD21, binds to the complement fragment C3d. It is primarily found on [[B cells]] and [[follicular dendritic cells]]. CR2 is involved in the enhancement of the humoral immune response by lowering the threshold for B cell activation.
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 ==
=== CR3 (Complement Receptor 3) ===
* [[Complement system]]
CR3, also known as CD11b/CD18 or Mac-1, binds to iC3b, a degradation product of C3b. It is expressed on [[macrophages]], [[neutrophils]], and [[natural killer cells]]. CR3 facilitates phagocytosis and cell adhesion.
* [[Humoral immunity]]
 
* [[Immune system]]
=== CR4 (Complement Receptor 4) ===
CR4, or CD11c/CD18, is similar to CR3 and also binds to iC3b. It is found on [[dendritic cells]], [[monocytes]], and [[granulocytes]]. CR4 is involved in phagocytosis and immune cell signaling.
 
== Function of Complement Receptors ==
Complement receptors mediate various immune functions, including:


== References ==
* '''Phagocytosis''': By binding to opsonized pathogens, complement receptors enhance the ability of phagocytes to engulf and destroy microbes.
{{Reflist}}
* '''Immune Complex Clearance''': Complement receptors on erythrocytes help transport immune complexes to the liver and spleen for removal.
* '''Cell Activation''': Binding of complement receptors can activate immune cells, leading to the production of cytokines and other inflammatory mediators.
* '''Regulation of Complement Activation''': Some complement receptors, like CR1, help regulate the complement cascade to prevent excessive inflammation and tissue damage.


== External links ==
== Clinical Significance ==
* {{MeshName|Complement+receptors}}
Dysfunction or deficiency of complement receptors can lead to immune system disorders. For example, defects in CR3 and CR4 are associated with [[leukocyte adhesion deficiency]], a condition characterized by recurrent infections due to impaired phagocyte function.


{{Complement system}}
== Related Pages ==
{{Pattern recognition receptors}}
* [[Complement system]]
* [[Immune system]]
* [[Phagocytosis]]
* [[Leukocyte adhesion deficiency]]


[[Category:Complement system]]
[[Category:Immunology]]
[[Category:Single-pass transmembrane proteins]]
[[Category:Receptors]]
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Latest revision as of 19:08, 22 March 2025

Receptors that bind complement proteins


Complement Receptors[edit]

Complement receptors are integral membrane proteins that interact with components of the complement system, a crucial part of the immune system. These receptors play a significant role in the immune response by recognizing and binding to complement proteins, thereby facilitating various immune functions such as phagocytosis, inflammation, and clearance of immune complexes.

Types of Complement Receptors[edit]

Complement receptors are classified based on the specific complement components they bind to. The major types include:

CR1 (Complement Receptor 1)[edit]

CR1, also known as CD35, is a receptor for the complement components C3b and C4b. It is expressed on the surface of erythrocytes, leukocytes, and kidney cells. CR1 plays a role in the clearance of immune complexes and the regulation of complement activation.

CR2 (Complement Receptor 2)[edit]

CR2, or CD21, binds to the complement fragment C3d. It is primarily found on B cells and follicular dendritic cells. CR2 is involved in the enhancement of the humoral immune response by lowering the threshold for B cell activation.

CR3 (Complement Receptor 3)[edit]

CR3, also known as CD11b/CD18 or Mac-1, binds to iC3b, a degradation product of C3b. It is expressed on macrophages, neutrophils, and natural killer cells. CR3 facilitates phagocytosis and cell adhesion.

CR4 (Complement Receptor 4)[edit]

CR4, or CD11c/CD18, is similar to CR3 and also binds to iC3b. It is found on dendritic cells, monocytes, and granulocytes. CR4 is involved in phagocytosis and immune cell signaling.

Function of Complement Receptors[edit]

Complement receptors mediate various immune functions, including:

  • Phagocytosis: By binding to opsonized pathogens, complement receptors enhance the ability of phagocytes to engulf and destroy microbes.
  • Immune Complex Clearance: Complement receptors on erythrocytes help transport immune complexes to the liver and spleen for removal.
  • Cell Activation: Binding of complement receptors can activate immune cells, leading to the production of cytokines and other inflammatory mediators.
  • Regulation of Complement Activation: Some complement receptors, like CR1, help regulate the complement cascade to prevent excessive inflammation and tissue damage.

Clinical Significance[edit]

Dysfunction or deficiency of complement receptors can lead to immune system disorders. For example, defects in CR3 and CR4 are associated with leukocyte adhesion deficiency, a condition characterized by recurrent infections due to impaired phagocyte function.

Related Pages[edit]

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