Mastocytosis, a type of mast cell disease, is a rare disorder affecting both children and adults caused by the accumulation of functionally defective mast cells (also called mastocytes) and CD34+ mast cell precursors.People affected by mastocytosis are susceptible to a variety of symptoms including itching, hives, and anaphylactic shock, caused by the release of histamine and other pro-inflammatory substances from mast cells.