Histiocytosis is a catch-all term for a group of diseases characterized by an excess of histiocytes, a type of cell from the immune system. All of these conditions as a whole are rare, affecting about 1 in every 200,000 people in total. They tend to occur more frequently in children than adults.
Two types of histiocytois that have been diagnosed on House, M.D. are Langerhans cell histiocytosis and Erdheim-Chester disease.