Langerhans cell histiocytosis is a disorder in which excess immune cells called Langerhans cells build up in the body. Langerhans cells, which help regulate the immune , are normally found throughout the body, especially in the skin, lymph nodes, spleen, lungs, liver, and bone marrow. In Langerhans cell histiocytosis, excess immature Langerhans cells usually form tumors called granulomas. However, Langerhans cell histiocytosis is not generally considered to be a form of cancer.

In approximately 80 percent of affected individuals, one or more granulomas develop in the bones, causing pain and swelling. The granulomas, which usually occur in the skull or the long bones of the arms or legs, may cause the bone to fracture.