Paroxysmal nocturnal hemoglobinuria (PNH) In this condition, the bone marrow--the soft spongy tissue that act as the blood manufacturing system for the entire body--produces defective red blood cells. The body's natural defense system then destroys these defective red blood cells in a process is known as hemolysis.
Healthy red blood cells help deliver oxygen and remove waste from the body. When red blood cells are damaged or destroyed, the result is acquired anemia, abnormal clotting (thrombosis), impaired bone marrow function, and a 3 to 5 percent chance of developing leukemia.
No one knows what causes this disorder.