XX/XY lymphoid chimerism in a boy with fatal lymphohistiocytic proliferation.

XX/XY chimerism, present in peripheral blood and bone marrow cells, but absent in other somatic tissues, was demonstrated in a 6-year-old patient. The initial clinical presentation with "butterfly" rash, pancytopenia with a hemolytic component, and hypergammaglobulinemia was suggestive of an immune disorder. Infiltration of skin, bone marrow, and lymph nodes with histiocytes, without disruption of the general configuration, was similar to the findings in familial lymphohistiocytosis. The results of specific cellular markers and of cytogenetic and immunologic studies are analyzed and the nature of the disease and origin of the chimeric state are discussed. The most likely explanations include a chronic graft-vs-host reaction induced by proliferation of foreign lymphoid cells derived from a blood transfusion that the child had received during infancy, or proliferation of neoplastic cells present in the transfused blood.