The exact cause is unknown. It is thought to be a disorder of the stem cell (the precursor of blood cells) in which a sensitivity to complement (a substance produced by the immune system) occurs in the cell membrane. The disease can affect people of any age. Red blood cell, white blood cell, and platelet counts may be low. Urine may be intermittently red or brown, signifying the breakdown of red blood cells with release of hemoglobin into the circulation and then the urine. Blood clots may form in some people. The disease may arise in relation to aplastic anemia, and may progress to acute myelogenous leukemia. Risk factors, except for prior aplastic anemia, are not known.
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