Also known as Systemic Mast Cell Disease, Systemic Mastocytosis, Urticaria pigmentosa. Mastocytosis (mas-toe-sigh-toe-sis) is a disorder caused by having too many mast cells in a person’s body. Mast cells are a kind of blood cell that are located in the skin, the linings of the stomach and intestine, and connective tissue (such as cartilage or tendons). Mast cells are important for survival. They help defend the skin, stomach, and intestine against disease. Mast cells are also involved in wound healing. Chemicals released by mast cells cause physiological changes that lead to typical allergic responses such as hives, itching, abdominal cramping, and even shock. When too many mast cells exist in a person’s body, the additional chemicals can cause bone pain, abdominal discomfort, nausea and vomiting, ulcers, diarrhea, skin lesions, and episodes of hypotension (very low blood pressure and faintness) or anaphylaxis (shock).