Paroxysmal nocturnal hemoglobinuria or paroxysmal nocturnal haemoglobinuria (
PNH), previously
Marchiafava–Micheli syndrome, is a rare, acquired, life-threatening disease of the blood characterized by
destruction of red blood cells by the
complement system, a part of the body's intrinsic immune system. This destructive process is a result of a defect in the formation of surface proteins on the red blood cell, which normally function to inhibit such immune reactions. Since the complement cascade attacks the red blood cells throughout the circulatory system, the hemolysis is considered an
intravascular hemolytic anemia. Other key features of the disease, notably the high incidence of
thrombosis, are not totally understood.