Paroxysmal nocturnal hemoglobinuria (PNH) 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.
PNH is the only hemolytic anemia caused by an acquired (rather than inherited) intrinsic defect in the cell membrane (deficiency of glycophosphatidylinositol leading to absence of protective proteins on the membrane). It may develop on its own ("primary PNH") or in the context of other bone marrow disorders such as aplastic anemia ("secondary PNH"). Only a minority (26%) have the telltale red urine in the morning that originally gave the condition its name.