What exactly is mastocytosis?

Too many mast cells: Mastocytosis is an overgrowth of mast cells, immune cells that release histamine and other chemicals upon activation. There are several forms, from small collections of cells in the skin (urticaria pigmentosa) to increased numbers in the bone marrow and throughout the body (systemic mastocytosis). Symptoms vary from hive-like reactions to recurrent anaphylaxis. See a specialist for more details.