It is a rare disease of the skin (the most common form), or of other parts of the body (very rare), like the stomach, the intestines and the bone marrow that is caused by having too many mast cells. It is the abnormal growth of mast cells in the body.
Mastocytosis (say mass-toe-sigh-toe-sis) is the abnormal growth of mast cells in the body. Mast cells are part of the immune system, which helps protect your body against infection. The most common form of mastocytosis is when mast cells accumulate on the skin, causing reddish-brown spots or bumps. In rare cases, mastocytosis can affect other parts of the body, such as the stomach, the intestines and the bone marrow. Mastocytosis can occur in people of any age. Mastocytosis (say “mas-toe-sigh-toe-sis”) is a rare disease of the skin (the most common form), or of other parts of the body (very rare), like the stomach, the intestines and the bone marrow. It's caused by having too many mast cells. Mast cells are a kind of blood cell. Mastocytosis can occur at any age.  Systemic mastocytosis (mas-to-sy-TOE-sis) is a rare disorder caused by a mutation that results in an excessive number of mast cells in your body. Mast cells normally help protect you from disease and aid in wound healing.  Systemic mastocytosis, often termed systemic mast cell disease (SMCD), is a myeloproliferative neoplasm characterized by infiltration of clonally derived mast cells in different tissues, including bone marrow (see the image below), skin, the gastrointestinal tract, the liver, and the spleen. Pathophysiology. Systemic mastocytosis (systemic mast cell disease) is characterized by mast cell infiltration of extracutaneous organs. Mast cells typically infiltrate the bone marrow and consequently affect the peripheral blood and coagulation system. Mastocytosis is a disorder that can occur in both children and adults. It is caused by the presence of too many mast cells in your body. You can find mast cells in skin, lymph nodes, internal organs (such as the liver and spleen) and the linings of the lung, stomach, and intestine. The most common cutaneous (skin) form is also called urticaria pigmentosa, which occurs when mast cells infiltrate the skin. Systemic mastocytosis is caused by mast cells accumulating in the tissues and can affect organs such as the liver, spleen, bone marrow, and small 1 Mastocytosis: A disorder where excessive amounts of mast cells proliferate in organs such as the skin, liver, bone, spleen and gastrointestinal tract. 2  Mast cells occur in connective tissue and defend the body against disease by releasing histamine to stimulate the immune system.  Mastocytosis is a disorder that can occur in both children and adults. It is caused by the presence of too many mast cells in your body. You can find mast cells in skin, lymph nodes, internal organs (such as the liver and spleen) and the linings of the lung, stomach, and intestine. Mast cells play an important role in helping your immune system defend these tissues from disease Overview. Mastocytosis is a condition that occurs when mast cells accumulate in skin and/or internal organs such as the liver, spleen, bone marrow, and small intestines. The signs and symptoms vary based on which part(s) of the body are affected. There are two main forms of mastocytosis: cutaneous and systemic What is mastocytosis. Mastocytosis is a group of rare disorders caused by the presence of too many mast cells, or (mastocytes), which are part of the immune system. 
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what is mastocytosis disease