Editing Complement system (section)

=== Classical pathway ===
{{Main|Classical complement pathway}}
[[File:Complement-pathways.png|350px|thumb|The classical and alternative complement pathways]]

The [[classical complement pathway|classical pathway]] is triggered by activation of the C1-complex. The '''C1-complex''' is composed of 1 molecule of [[Complement component 1q|C1q]], 2 molecules of C1r and 2 molecules of C1s, or ''C1qr<sup>2</sup>s<sup>2</sup>''. This occurs when C1q binds to [[IgM]] or [[Immunoglobulin G|IgG]] complexed with [[antigen]]s. A single pentameric IgM can initiate the pathway, while several, ideally six, IgGs are needed. This also occurs when [[Complement component 1q|C1q]] binds directly to the surface of the pathogen. Such binding leads to conformational changes in the C1q molecule, which leads to the activation of two [[Complement component 1r|C1r]] molecules. C1r is a serine protease. They then cleave [[Complement component 1s|C1s]] (another serine protease). The C1r<sup>2</sup>s<sup>2</sup> component now splits [[Complement component 4|C4]] and then [[Complement component 2|C2]], producing C4a, C4b, C2a, and C2b (historically, the larger fragment of C2 was called C2a but is now referred to as C2b). C4b and C2b bind to form the classical pathway C3-convertase (C4b2b complex), which promotes cleavage of C3 into C3a and C3b. C3b later joins with C4b2b to make C5 convertase (C4b2b3b complex).<ref>{{Cite web |title=Classical Pathway (CP) |url=http://www.complementsystem.se/classical-pathway |url-status=dead |archive-url=https://web.archive.org/web/20160602151601/http://www.complementsystem.se/classical-pathway |archive-date=2 June 2016 |access-date=6 June 2022 |website=www.complementsystem.se |publisher=Euro Diagnostica}}</ref>