CD1A and Langerhans cell histiocytosis: Langerhans histiocytosis, also known as histiocytosis X, Letterer-Siwe disease or eosinophilic granuloma, is a rare disease defined as a proliferation of certain cells of the mononuclear system known as Langerhans cells, which contain intra-cytoplasmic Birbeck granules and display S100 protein and CD1a antigenic markers on their surface [5].