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Aplastic anaemia may result from defects in haemopoietic stem cells or in the marrow micro-environment. The precise mechanism is often unknown but the ability to correct the defect by bone marrow transplantation suggests that the fault often lies in the stem cells.
Many individuals show evidence of an immune-mediated suppression of haematopoiesis. Cellular mechanisms seem most important. In some individuals T cells produce in vitro suppression of autologous marrow stem cells. Humoral mechanisms may be involved since antibodies to stem cells have been found.
Additional support for the importance of immunosuppression derives from transplant studies. Only 50% of those treated by transplantation from an identical twin donor recover normal haematopoiesis; preceding the second transplant with intensive immunosuppression using cyclophosphamide then facilitates full recovery. Additionally, 50% of patients treated with anti-thymocyte globulin (ATG) or anti - lymphocyte globulin (ALG) recover without marrow transplantation.
Failure of a 'committed' cell may result in decreased cell production or renewal, resulting in deficiencies such as pure red cell aplasia.