Infantile histiocytosis X.

Skin biopsies and a lymphnode of three children with infantile Histiocytosis-X (Letter-Siwe Disease) were studied with enzymehistochemical and sheep-erythrocyte rosetting techniques. The majority of cells making up the infiltrates of skin and lymphnode showed rather weak acid phosphatase and nonspecific esterase activity but considerable leucyl-beta-naphtylamidase activity. Sheep-erythrocyte rosetting techniques performed on frozen sections indicated the presence of receptors for the Fc fragment of IgG, but no receptors for C3 could be demonstrated. Cells with the same enzymehistochemical characteristics could be found in thymus-dependent areas of normal spleen, of normal and reactive lymphnodes and in thymic medulla but not in B-cell areas or thymic cortex. It is suggested that Histiocytosis-X cells belong to the Mononuclear Phagocyte System and that they are related to or identical with cells normally present in the thymus dependent areas of the lymphoid tissue involved with the functioning of cell-mediated immunity.