A drug used to prevent red blood cells from being destroyed in patients with a rare red blood cell disorder called paroxysmal nocturnal hemoglobinuria (PNH). It is also used to treat another rare disorder called atypical hemolytic urea syndrome (aHUS), in which blood clots form in small blood vessels. Eculizumab binds to an immune system protein called C5. This helps keep red blood cells from breaking down and helps keep blood clots from forming. Eculizumab is a type of monoclonal antibody.