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Biology and management of acquired severe aplastic anemia.

Severe aplastic anemia (SAA) is a life-threatening bone marrow failure disorder characterized by pancytopenia and a hypocellular marrow. Drugs, chemical exposure, radiation, and viruses are implicated as etiologic agents, although the majority of community-acquired SAA is idiopathic. Regardless of the inciting event, most cases of SAA result from immune-mediated destruction of bone marrow progenitor cells, which spares pluripotent hematopoietic stem cells. SAA is treated by either allogeneic bone marrow transplantation (BMT) or immunosuppressive therapy. BMT restores normal hematopoiesis and cures the disease in 60% to 80% cases, with the major causes of failure being graft rejection and graft-versus-host disease. Most patients treated with immunosuppressive therapy recover hematopoiesis sufficiently to not require transfusions and are free of infection, although in many, recovery is incomplete. Moreover, up to 50% of SAA patients successfully treated with immunosuppressive therapy relapse or develop a secondary clonal disorder, such as paroxysmal nocturnal hemoglobinuria, myelodysplastic syndrome, or leukemia. High-dose cyclophosphamide without BMT is capable of restoring normal hematopoiesis with little or no risk of relapse or secondary clonal disorders. A number of effective treatment options for the treatment of SAA are now available. The optimal approach for definitive management of SAA continues to evolve.