Classical complement activations. IgM and IgG3, IgG1, and IgG2 activate the classical pathway of ...

Classical complement activations. IgM and IgG3, IgG1, and IgG2 activate the classical pathway of complement after a conformational change caused by antigen binding. C1q binds to the bound antibody and is part of the Ca+2 ion-dependent C1 molecular complex of C1q, C1r, and C1s. When 2 of the globular heads of the C1q molecule are bound C1q activates C1r and then C1r activates C1s. Next C1s cleaves C4 into C4a and C4b. C4a is released into the fluid phase and C4b lands on the cell surface. On the cell surface, C4b forms a Mg+2 ion-dependent complex with C2; C2 is then cleaved by C1s, forming C2a (the larger piece), which lands on the surface, and C2b (the smaller piece), which floats away. This brings us to the C3 convertase of the classical pathway C4b2a.