Etiologic mechanisms of hematopoietic failure

Our understanding of the pathophysiology of aplastic anemia has lagged behind advances in treatment of the disease. The data available indicate that the heterogeneous assortment of drugs, as well as the array of chemical, physical, and infectious agents that are clinically associated with aplastic anemia, probably exert their action through restricted pathways. Based on certain clinical features of aplastic anemia, animal models of hematopoietic failure, and considerations of the organization of the hematopoietic system, we propose two general types of aplastic anemia as a conceptual framework for further studies: type I aplastic anemia, representing a congenital or acquired stem cell defect that can only be cured by bone marrow transplantation, and type II aplastic anemia, which is caused by reversible suppression of normal stem cells by external agents and is amenable to immunosuppressive therapy.