1-3. Formation of recognition unit. 1. Activation of C1 by an lgG doublet. 2. Step one;
activation of C4 by C1. 3. Step two; activation of C2 by C1, to form C4b2a.
4. Cleavage of C3 molecule leading to cell-bound C3b. 5-8. Formation of membrane
attack unit; C4b2a3b cleaves C5 molecule. Cell-bound C5b absorbs C6, C7, and C8,
leading to membrane defects, thus allowing ions to enter cell. 9. Membrane ruptures
releasing hemoglobin from cell. (Courtesy of G. Garratty.)
Figure 1-4. The complement cascade (classic pathway).