The membrane attack complex (MAC), the end-product of the terminal complement system, normally does not attack the body's own red blood cells, because CD59 prevents this. However, patients with paroxysmal nocturnal haemoglobinuria (PNH) lack CD59, which allows MAC to attack the red blood cells and cause lysis Eculizumab binds complement C5, thereby inhibiting the entire terminal complement system including the generation of the MAC. In PNH patients this results in a strong decrease in haemolysis, but also reduced clearance of Neisseria