Systemic mastocytosis is a rare disorder in which too many mast cells accumulate in the body. Mast cells are a type of white blood cell that helps the body fight infection and is involved in allergic reactions. In systemic mastocytosis, the mast cells accumulate in the skin, bone marrow, and other organs, leading to a variety of symptoms. These can include skin rashes, flushing, abdominal pain, nausea, vomiting, diarrhea, and fatigue. Treatment for systemic mastocytosis may include medications to reduce the number of mast cells, as well as medications to reduce symptoms.