Systemic mastocytosis (mas-to-sy-TOE-sis) is a rare disorder that results in too many mast cells building up in your body. A mast cell is a type of white blood cell. Mast cells are found in connective tissues throughout your body. Mast cells help your immune system function properly and normally help protect you from disease. When you have systemic mastocytosis, excess mast cells build up in your skin, bone marrow, digestive tract or other body organs. When triggered, these mast cells release substances that can cause signs and symptoms similar to those of an allergic reaction and, sometimes, severe inflammation that may result in organ damage. Common triggers include alcohol, spicy foods, insect stings and certain medications.