The C1 complex is composed of three complement proteins called C1q, C1r, and C1s. The C1q is the portion of the C1 complex that can bind to the Fc portion of the antibodies IgG or IgM, certain microbes, or the C-reactive protein (CRP) from the acute phase response.
In the case of antibodies, the Fab portions of 2 molecules of IgG (or 1 molecule of IgM) bind to their corresponding epitopes on an antigen. Binding of the IgG to the epitopes activates the Fc portion of the IgG enabling C1q of the C1 complex to bind. The binding of C1q to the antibody molecules activates the C1r portion of C1 which, in turn, activates C1s. This activation gives C1s enzymatic activity to cleave complement protein C4 into C4a and C4b and complement protein C2 into C2a and C2b.