Classical complement pathway
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The classical complement pathway can be initiated by the binding of antigen-antibody complexes to the C1q protein. The globular regions of C1q recognize and bind to the Fc region of antibody isotypes IgG or IgM.cite-ref-complement-in-disease-2-2[2] These globular regions of C1q can also bind to bacterial and viral surface proteins, apoptotic cells, and acute phase proteins.cite-ref-5[5] 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.cite-ref-overview-of-complement-1-2[1]cite-ref-c1q-4-1[4]
Formation of C4b convertase
The binding of C1q with pathogen surface or antigen-antibody immune complex leads to conformational changes and the activation of the serine protease C1r. The activated C1r then cleaves and activates the serine protease C1s.cite-ref-complement-history-3-2[3]cite-ref-c1q-4-2[4] Activated C1s cleaves C4 into C4a and C4b.
Regulation of C4b