Mastocytosis is a group of disorders in both children and adults caused by the presence of too many mast cells in a person's body. Located in skin, the linings of the stomach and intestine, as well as other sites, mast cells may play an important role in helping defend these tissues from disease. By releasing chemical "alarms" such as histamine, mast cells attract other key players of the immune defense system to areas of the body where they are needed.

Chemicals released by mast cells cause changes in the immune system leading to typical allergy symptoms such as itching, abdominal cramping, and even anaphylaxis (dangerously low blood pressure).