Mastocytosis (urticaria pigmentosa) is a mostly harmless condition where your child has more than the normal number of mast cells in the body. Mast cells are found in everyone in the skin, bowels, gut (stomach), lungs and air passages. Mast cells do not usually cause problems. In fact, they play an important role in helping our body's immune system to protect the body against diseases. A person with mastocytosis in the skin will have pink or brown flat spots that will eventually go away after several years. Some people will have only a few of these spots while others will have many.