Complement regulation and eculizumab. The lectin, classical, and alternative pathways converge at the point of C3 activation. In PNH, hemolysis is usually chronic because the alternative pathway is always in a low-level activation state through a process known as tick-over. Terminal complement begins with cleavage of C5 to C5a and C5b. C5b oligomerizes with C6, C7, C8, and multiple C9 molecules to form the MAC. CD55 inhibits proximal complement activation by blocking the formation of C3 convertases; CD59 inhibits terminal complement activation by preventing the incorporation of C9 into the MAC. The absence of CD55 and CD59 on PNH cells leads to hemolysis, inflammation, platelet activation, and thrombosis. Eculizumab inhibits terminal complement activation by binding to C5 and preventing generation of C5a and C5b.