is a rare condition that can affect many different organs of the body. This condition, which usually affects adults, is characterized by excessive production and accumulation of histiocytes (specific cells that normally play a role in responding to infection and injury) within multiple tissues and organs. As a result, these tissues and organs become thickened, dense and fibrotic. Sites of involvement may include the long bones, skin, tissues behind the eyeballs, lungs, brain, and pituitary gland, among others. Signs and symptoms, as well as disease course, depend on the specific location and extent of involvement. Without successful treatment, organ failure can occur. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.