Editing Classical complement pathway (section)

===Initiation===
The classical complement pathway can be initiated by the binding of antigen-antibody complexes to the [[Complement component 1q|C1q]] protein. The globular regions of C1q recognize and bind to the [[Fragment crystallizable region|Fc]] region of antibody isotypes IgG or IgM.<ref name="Complement in disease">{{cite journal|last1=Vignesh|first1=Pandiarajan|last2=Rawat|first2=Amit|last3=Sharma|first3=Madhubala|last4=Singh|first4=Surjit|title=Complement in autoimmune diseases|journal=Clinica Chimica Acta|date=February 2017|volume=465|pages=123–130|doi=10.1016/j.cca.2016.12.017|pmid=28040558}}</ref> These globular regions of C1q can also bind to bacterial and viral surface proteins, apoptotic cells, and acute phase proteins.<ref>{{Cite book| title = Advances in Immunology Volume 46| last1 = Ahearn| first1 = Joseph M.| last2 = Fearon| first2 = Douglas T.| chapter = Structure and Function of the Complement Receptors, CR1 (CD35) and CR2 (CD21)| date = 1989-01-01| editor-last = Dixon| editor-first = Frank J.| volume = 46| pages = 183–219| doi = 10.1016/s0065-2776(08)60654-9| pmid = 2551147| isbn = 9780120224463}}</ref> In the absence of these activation factors, C1q is part of the inactive C1 complex which consists of six molecules of C1q, two molecules of [[C1r]], and two molecules of [[C1s]].<ref name="Overview of Complement" /><ref name="C1q" />