Systemic mastocytosis, often termed systemic mast cell disease (SMCD), is a clonal disorder of the mast cell and its precursor cells. The clinical symptoms and signs of systemic mastocytosis (systemic mast cell disease) are due to the accumulation of these clonally derived mast cells in different tissues, including bone marrow, skin, the gastrointestinal (GI) tract, the liver, and the spleen.