Histiocytosis X is a clonal proliferation of cutaneous dendritic (Langerhans) cells, with their characteristic CD1a antigen expression and typical Birbeck granules, which may be observed ultramicroscopically. Tumorous forms of Langerhans cells can be found at various locations (bones, parenchymal organs), which is probably caused by presence of abnormal chemokine receptors that attract the cells into these organs. chemokinových receptorů, které je atrahují do příslušných orgánů.

Normally, dendritic cells are distributed in the epidermis. After antigen stimulation they change and lose the CD1 antigen as well as the Birbeck granules. Then they migrate into the lymph nodes, where they present the antigen to the T4 lymphocytes.