Thymus in untreated systemic histiocytosis X.

Histology, ultrastructure, and lymphocyte subsets were evaluated in thymic tissue obtained from a child with disseminated histiocytosis X (HX) prior to therapy. Extensive medullary infiltration by typical HX cells was associated with active destruction of thymic epithelium, medullary pseudocysts, and prevalence of calcospherites, some of which were in HX cells. The presence of minimally affected Hassall bodies indicates that the thymus was probably normal prior to onset of disease. Conventional studies of immunologic function showed no deficits, but reduction of OKT6- and OKT8-labeled lymphocytes in a thymic suspension may mean that thymic processing of lymphocytes was impaired. Destruction of thymic epithelium occurs regularly in HX, but it may be an insignificant epiphenomenon resulting from the affinity of HX cells for squamous epithelium.