A group of rare disorders in which too many histiocytes (a type of white blood cell) build up in certain tissues and organs, including the skin, bones, spleen, liver, lungs, and lymph nodes. This can cause damage to tissue or tumors to form in one or more parts of the body. The tumor may be benign (not cancer) or malignant (cancer). There are different types of histiocytosis, including Langerhans cell histiocytosis (LCH), hemophagocytic lymphohistiocytosis (HLH), and malignant histiocytic disorders. Histiocytosis occurs most often in children, but can occur at any age.