Abnormal accumulation of mast cells within one or more organs. Mast cells are a type of tissue cell found throughout the body that release chemicals as part of the body's normal response to injury but sometimes as part of an allergic response. Cutaneous mastocytosis is a benign disease of the skin, usually affecting children. Systemic mastocytosis affects mostly adults, who may experience signs and symptoms related to the organs affected such as skin rashes or characteristic red, blistering lesions, peptic ulcers, chronic diarrhea, joint pain or enlargement of the liver, spleen, or lymph nodes. Systemic mastocytosis may progress slowly or may be aggressive, causing organ dysfunction and, in rare cases, causing a form of leukaemia.