C5 and paroxysmal nocturnal hemoglobinuria: Eculizumab and ravulizumab, humanized monoclonal antibodies blocking cleavage of C5, have been clinically applied to two disorders, paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome, in which activation of the complement system by abnormal complement regulatory mechanisms destroys autologous cells [18, 20].