<prompt> Histiocytosis Xis seen in except-Hand schuller Christian diseaseEosinophilic granulomaLetter-siwe syndromeTorres syndrome  <response>: Ans. is d i.e., Torres syndromeLangerhans cell histiocytosis Histiocytosis X o Histiocytosis X is characterized by proliferation of Langerhans histiocytes tissue macrophages. o It is Subdivided into three Categories:-i Letterer-siwe syndromeii Hand-Schuller-Christian diseaseiii Eosinophilic granuloma.o Tumor cells in each are derived from dendritic cells and express S-100, CD la and HLA-DR.o The presence of Birbeck granules in the cytoplasm is characteristic.o Under electron microscope, Birbeck granules have a pentalaminar, rod like, tubular appearance and sometimes a dilated terminal end tennis-rachet appearance. 