The etiology of aplastic anemia is complex, and about 50% of all cases are idiopathic. There has been considerable speculation about the possible role of an immunologic mechanism in the pathogenesis of some cases, but this mechanism has not yet been fully documented. Evidence for such a mechanism is described in the case of a woman of 26 whose bone marrow was shown by multiple criteria to have a block in early myeloid differentiation. This block was overcome in vitro by elimination of marrow lymphocytes. Furthermore, this differentiation block was transferred in vitro to normal marrow by coculturing with the patient's marrow. It is suggested that some cases of aplastic anemia may be due to an immunologically based suppression of marrow cell differentiation rather than to a defect in stem cells or their necessary inductive environment.
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