Activation of the C1 complex initiates the
classical complement pathway. This occurs when
C1q binds to
antigen-antibody complexes. The
antibodies IgM or certain subclasses of
IgG complexed with
antigens are able to initiate the complement system: a single
pentameric IgM can initiate the pathway, while several
monomeric IgG molecules are needed. C1q can also be activated in other ways, for example by binding to
pentraxins such as
C-reactive protein or directly to the surface of pathogens. Such binding of C1q leads to conformational changes in the C1q molecule, which activates the associated
C1r molecules. Active C1r cleaves the
C1s molecules, activating them. Active C1s splits
C4 and then
C2, producing C4a, C4b, C2a and C2b. The classical pathway
C3-convertase (C4bC2b complex) is created, which promotes cleavage of
C3. == References ==