Mastocytosis, also known as mast cell disease, is an uncommon disorder caused by the accumulation of a normally rare white blood cell type called mast cells. Normal mast cells are involved in fighting certain infections but are most well known as the primary cells that drive allergic responses. When someone has abnormal accumulations of mast cells in the skin, bone marrow, and internal organs (liver, spleen, gastrointestinal tract and lymph nodes), this is caused mastocytosis. In most adult patients, mastocytosis tends to be persistent, and may progress into a more advanced category in a minority of patients. Mastocytosis can cause problems because factors produced by the abnormal mast cells can scar and damage the host organs.