<prompt> Histiocytosis X is seen in except -Hand schuller Christian diseaseEosinophilic granulomaLetter-siwe syndromeTorres syndrome  <response>: Ans. is d i.e., Torres syndrome Langerhans cell histiocytosis Histiocvtosis X C. Histocytosis X is characterized by proliferation of Langerhans histiocytes tissue macrophages. o It is subdivided into three categories : - i Letterer - Siwe syndrome ii Hand -Schuller-Christian disease iiiEosinophilic granuloma. o Tumor cells in each are derived from dendritic cells and express S-100, CD1 a and HLA-DR. o The presence of Birbeck granules in the cytoplasm is characteristic. o Under the electron microscope, Birbeck granules have a pentalaminar, rodlike, tubular apperance and sometimes a dilated terminal end tennis - racket appearance. 