The classical pathway (CP) is initiated when immune complexes are formed after IgG or IgM binding to pathogens or to other foreign and non-self antigens. The C1 complex, a multimeric complex consisting of C1q, C1r and C1s molecules, then binds to the Fc portion of the IgG or IgM immune complex. Activation of C1s and C1r occurs as a consequence of C1q binding to the exposed Fc portion of IgG or IgM. C1s then cleaves C4 and C2 to form the CP C3 convertase, C4bC2a.