C1 complex

It is the first component of the classical complement pathway and is composed of the subcomponents C1q, C1r and C1s.

[2][3][4] Activation of the C1 complex initiates the classical complement pathway.

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.

[3] C1q can also be activated in other ways, for example by binding to pentraxins such as C-reactive protein[2] or directly to the surface of pathogens.

The classical pathway C3-convertase (C4bC2b complex) is created, which promotes cleavage of C3.