Histiocytosis -X of the Thymus

Abstract

Histiocytosis — X [H-X, “Langerhans’ cell histiocytosis”] is a nonneoplastic but often biologically aggressive proliferation of Langerhans’ cells [LC] which primary function is an uptake, processing and presentation of an antigen in squamous epithelia.^5,7,8,27 Despite observations of impaired immunity in H-X, the cause of H-X remain largely unknown and its neoplastic potential is practically indeterminable upon histopathology alone.⁸ The following main non-histopathologic criteria for predicting a poor prognosis have been found: [i] organ involvement which has to be assessed on the basis of a score system; [ii] dysfunction of organs that are involved in the diseases; [iii] rapid progression and poor response to therapy and finally [iv] very young age, especially when related to pediatric patients.^{3,12,15,21,25,27} Thymic primary presentation of H-X is rare. No LC are found in normal thymus, instead corresponding function is ascribed to interdigitating cells (IDC) which constitute a cellular substrate for intrathymic negative selection process.^26,29,30,31 Both LC and IDC are believed to develop from common ancestral cells based within the bone marrow monocytic precursor compartment.