Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. X-type histiocytoses are a clinically well-defined group of cutaneous syndromes characterized by infiltrates of Langerhans cells, as opposed to Non-X histiocytosis in which the infiltrates contain monocytes/macrophages. Monocyte is a type of white blood cell, part of the human body''s immune system. Monocytes have two main functions in the immune system: (1) replenish resident macrophages and dendritic cells under normal states, and (2) in response to inflammation signals, monocytes can move quickly (approx. 8-12 hours) to sites of infection in the tissues and divide/differentiate into macrophages and dendritic cells to elicit an immune response. Half of them are stored in the spleen. Monocytes are usually identified in stained smears by their large bilobate nucleus.