PNH presents such a scenario wherein the reappearance of even low levels of free C5 could result in an intravascular hemolytic crisis, particularly in patients with a large proportion of PNH red blood cells, or result in thrombosis owing to both intravascular hemolysis and complement activation of PNH white blood cells and platelets. Here, C5 is linked to paroxysmal nocturnal hemoglobinuria.