Mastocytosis (say mass-toe-sigh-toe-sis) is the abnormal growth of mast cells in the body. Mast cells are part of the immune system, which helps protect your body against infection. The most common form of mastocytosis is when mast cells accumulate on the skin, causing reddish-brown spots or bumps.In rare cases, mastocytosis can affect other parts of the body, such as the stomach, the intestines and the bone marrow. Mastocytosis can occur in people of any age.he most common form of mastocytosis is when mast cells accumulate on the skin, causing reddish-brown spots or bumps. In rare cases, mastocytosis can affect other parts of the body, such as the stomach, the intestines and the bone marrow. Mastocytosis can occur in people of any age.