Mastocytosis is an abnormal accumulation of mast cells in the skin and sometimes in various other parts of the body.

Mastocytosis is rare. It differs from typical allergic reactions because it is chronic rather than episodic. Mastocytosis develops when mast cells increase in number and accumulate in tissues over a period of years. Mast cells, a component of the immune system, produce histamine, a substance involved in allergic reactions and the production of stomach acid. Because the number of mast cells increases, levels of histamine increase.

There are three main forms of mastocytosis. In a rare form, mast cells accumulate as a single mass in the skin (mastocytoma). Typically, a mastocytoma develops before age 6 months. In a form called urticaria pigmentosa, mast cells accumulate in many areas of the skin, forming small reddish brown spots or bumps. Rarely, urticaria pigmentosa progresses to systemic mastocytosis during adulthood. In systemic mastocytosis, mast cells accumulate in the skin, stomach, intestines, liver, spleen, lymph nodes, and bones.

Symptoms and Diagnosis

A single mastocytoma does not cause symptoms. Rubbing or scratching the spots of urticaria pigmentosa may make the spots itch. Itching may be worsened by changes in temperature, contact with clothing or other materials, or use of some drugs. Consuming hot beverages, spicy foods, or alcohol may also make itching worse. Rubbing or scratching the spots may result in hives and make the skin turn red. Flushing and widespread reactions, including anaphylactic reactions, may occur.

Systemic mastocytosis causes itching and flushing. It can cause widespread reactions, which tend to be severe and include anaphylactoid reactions. Anaphylactoid reactions resemble anaphylactic reactions, but no allergen triggers them. Bone pain and abdominal pain are common. Peptic ulcers and chronic diarrhea may develop because the stomach produces too much histamine, which stimulates the production of stomach acid.

Urticaria pigmentosa may be diagnosed based on the presence of the typical spots that, when scratched, result in hives and redness. A biopsy may also be performed. If mastocytosis affecting the skin is suspected, a sample of skin tissue is removed and examined under a microscope for mast cells. If systemic mastocytosis is suspected, a sample is taken from the bone marrow or other tissues.

Anaphylactoid Versus Anaphylactic Anaphylactoid reactions resemble anaphylactic reactions. However, anaphylactoid reactions may occur after the first exposure to a substance—for example, after the first injection of certain drugs, such as polymyxin, pentamidine, opioids, or the radiopaque dyes sometimes used with x-ray procedures. Anaphylactoid reactions are not allergic reactions because IgE, the class of antibodies involved in allergic reactions, does not cause them. Rather, the reaction is caused by the substance itself. Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) can cause anaphylactoid reactions in some people, particularly those with year-round allergic rhinitis and nasal polyps. If possible, doctors avoid using dyes with x-ray procedures in people who have anaphylactoid reactions to such dyes. However, some disorders cannot be diagnosed without dyes. In such cases, special dyes that reduce the risk of reactions are used. In addition, drugs that block anaphylactoid reactions, such as prednisone, diphenhydramine, or ephedrine, are usually given before the dye is injected.

Treatment

In children, a mastocytoma usually disappears spontaneously. Itching due to urticaria pigmentosa may be treated with antihistamines. Systemic mastocytosis is treated with antihistamines and histamine-2 (H₂) blockers (which reduce acid production in the stomach (see Drugs Used to Treat Peptic Disorders)). Cromolyn given by mouth can relieve digestive problems. A self-injecting syringe of epinephrine should always be carried for prompt emergency treatment of anaphylactic reactions. Ultraviolet light and corticosteroid creams applied to the skin may be used to treat the skin symptoms of mastocytosis.

Last full review/revision February 2003