Schematic representation of the complement cascade reaction in healthy individuals, in patients with PNH, and in patients with PNH receiving the C5-inhibiting antibody. In humans the complement system consists of more than 30 plasma and cell-surface proteins. The complement system may be activated by 3 different pathways: the classic, lectin, and alternative pathways. The primary goal of the activation pathway is C3b deposition on the target cell, which is followed by the formation of the membrane attack complex (MAC) in the lytic pathway of complement activation, necessary for the lysis of the target. The deficiency of the surface proteins CD55 and CD59 leads to the uncontrolled lysis of PNH red cells by complement. The C5-binding antibody eculizumab blocks the lytic pathway of complement activation.