Editing Complement system (section)

== Activation of complements by antigen-associated antibody ==
In the classical pathway, C1 binds with its C1q subunits to Fc fragments (made of CH2 region) of IgG or IgM, which has formed a complex with antigens. C4b and C3b are also able to bind to antigen-associated IgG or IgM, to its Fc portion.<ref name="abbas" /><ref name="Roitt" /><ref name="murphy2008" />

Such immunoglobulin-mediated binding of the complement may be interpreted as that the complement uses the ability of the immunoglobulin to detect and bind to non-self antigens as its guiding stick. The complement itself can bind non-self pathogens after detecting their [[pathogen-associated molecular patterns]] (PAMPs),<ref name="murphy2008" /> however, utilizing specificity of the antibody, complements can detect non-self targets much more specifically.{{citation needed|date=July 2022}}

Some components have a variety of binding sites. In the classical pathway, C4 binds to Ig-associated C1q and C1r<sup>2</sup>s<sup>2</sup> enzyme cleaves C4 to C4b and 4a. C4b binds to C1q, antigen-associated Ig (specifically to its Fc portion), and even to the microbe surface. C3b binds to antigen-associated Ig and to the microbe surface. Ability of C3b to bind to antigen-associated Ig would work effectively against antigen-antibody complexes to make them soluble.{{citation needed|date=May 2015}}