Granuloma (plural granulomas or granulomata) is an inflammation found in many diseases. It is a collection of immune cells known as histiocytes. Granulomas form when the immune system attempts to wall off substances it perceives as foreign but is unable to eliminate. Such substances include infectious organisms including bacteria and fungi, as well as other materials such as keratin and suture fragments. The adjective granulomatous means characterized by granulomas.