Editing Classical complement pathway (section)

{{short description|Aspect of the immune system}}
[[File:Complement pathways.png|thumb|393x393px|Classical and alternative pathways shown with their corresponding proteins]]

The '''classical complement pathway''' is one of three pathways which activate the [[complement system]], which is part of the [[immune system]]. The classical complement pathway is initiated by [[antigen-antibody complex]]es with the antibody isotypes [[Immunoglobulin G|IgG]] and [[Immunoglobulin M|IgM]].<ref name="Overview of Complement" /><ref name="Complement in disease" />

Following activation, a series of [[protein]]s are recruited to generate [[C3-convertase|C3 convertase]] (C4b2b, [[Complement system#Complement protein fragment nomenclature|historically]] referred C4b2a), which cleaves the [[Complement component 3|C3]] protein. The [[C3b]] component of the cleaved C3 binds to C3 convertase (C4b2b) to generate [[C5-convertase|C5 convertase]] (C4b2b3b), which cleaves the [[complement component 5|C5]] protein. The cleaved products attract phagocytes to the site of infection and tags target cells for elimination by phagocytosis.  In addition, the C5 convertase initiates the terminal phase of the complement system, leading to the assembly of the membrane attack complex ([[Complement membrane attack complex|MAC]]). The membrane attack complex creates a pore on the target cell's membrane, inducing cell lysis and death.<ref name="Complement in disease" /><ref name="Complement history" />

The classical complement pathway can also be activated by [[apoptotic]] cells, necrotic cells, and [[acute phase protein]]s.<ref name="Overview of Complement">{{cite journal|last1=Noris|first1=Marina|last2=Remuzzi|first2=Giuseppe|title=Overview of Complement Activation and Regulation|journal=Seminars in Nephrology|date=November 2013|volume=33|issue=6|pages=479–492|doi=10.1016/j.semnephrol.2013.08.001|pmc=3820029|pmid=24161035}}</ref><ref name="Complement history">{{cite journal|last1=Nesargikar|first1=Prabhu|last2=Spiller|first2=B.|last3=Chavez|first3=R.|title=The complement system: History, pathways, cascade and inhibitors|journal=European Journal of Microbiology & Immunology|date=June 2012|volume=2|issue=2|pages=103–111|doi=10.1556/EuJMI.2.2012.2.2|pmc=3956958|pmid=24672678}}</ref><ref name="C1q">{{cite journal|last1=Thielens|first1=Nicole M.|last2=Tedesco|first2=Francesco|last3=Bohlson|first3=Suzanne S.|last4=Gaboriaud|first4=Christine|last5=Tenner|first5=Andrea J.|date=June 2017|title=C1q: A fresh look upon an old molecule|journal=Molecular Immunology|doi=10.1016/j.molimm.2017.05.025|pmid=28601358|pmc=5582005|volume=89|pages=73–83}}</ref>