C5 and paroxysmal nocturnal hemoglobinuria: One central part of the complement system is C5, and eculizumab is a C5 blocking monoclonal antibody approved for the treatment of paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS), two rare disease caused by an autoimmune activation.72, 73 The drug is dosed by infusion at very high doses, up to 1200 mg every second week.