Diseases & Conditions



Mastocytosis is an uncommon abnormal accumulation of mast cells in the skin and sometimes in various other parts of the body. People may have itchy spots and bumps, flushing, digestive upset, and sometimes bone pain. Symptoms suggest the diagnosis, and a biopsy of the skin or bone marrow can confirm it. If mastocytosis affects only the skin, it may resolve on its own, but if it affects other parts of the body, it cannot be cured. Antihistamines help relieve itching, and histamine-2 (H 2 ) blockers help relieve digestive upset.

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. What causes the disorder is unclear.

Mastocytosis may affect primarily the skin (called cutaneous mastocytosis) or other parts of the body (called systemic mastocytosis). Cutaneous mastocytosis: This form usually occurs in children. Occasionally, mast cells accumulate only as a single mass in the skin (mastocytoma), typically before age 6 months. More commonly, mast cells congregate in many areas of the skin, forming small reddish brown spots or bumps (called urticaria pigmentosa). Urticaria pigmentosa only rarely progresses to systemic mastocytosis in children but may do so more often in adults. Systemic mastocytosis: This form usually occurs in adults. Mast cells accumulate in the skin, stomach, intestine, liver, spleen, lymph nodes, and bone marrow (where blood cells are produced). Organs may continue to function, with little disruption. But if many mast cells accumulate in the bone marrow, too few blood cells are produced, and serious blood disorders, such as leukemia, can develop. If many mast cells accumulate in organs, the organs malfunction. The resulting problems can be life threatening.


A single mastocytoma does not cause symptoms. Spots and bumps may itch, particularly if they are rubbed or scratched. Itching may be worsened by changes in temperature, contact with clothing or other materials, or use of some drugs (including nonsteroidal anti-inflammatory drugs). Consuming hot beverages, spicy foods, or alcohol or exercising may also make itching worse. Rubbing or scratching the spots may result in hives and make the skin turn red.

Flushing is common. Peptic ulcers may develop because too much histamine is produced, stimulating secretion of excess stomach acid. Ulcers can cause stomach pain. Nausea, vomiting, and chronic diarrhea may also occur. The abdomen may enlarge if the liver and spleen malfunction, causing fluid to accumulate. If bone marrow is affected, bone pain can result.

Widespread reactions, including anaphylactic reactions, may occur. With systemic mastocytosis, the widespread reactions tend to be severe. They include anaphylactoid reactions, which cause fainting and a life-threatening drop in blood pressure (shock). Anaphylactoid reactions resemble anaphylactic reactions, but no allergen triggers them.

Systemic mastocytosis may affect the bone marrow, and up to 30% of adult patients with systemic mastocytosis develop cancers, particularly myelocytic leukemias. In these people, life expectancy may be shortened.


Doctors suspect the diagnosis based on symptoms, particularly spots that, when scratched, result in hives and redness. A biopsy can confirm the diagnosis. Usually, a sample of skin tissue is removed and examined under a microscope for mast cells. Sometimes a sample is taken from the bone marrow. Blood tests to measure levels of chemical substances related to mast cells are done. High levels support the diagnosis of systemic mastocytosis.

Anaphylactoid Versus Anaphylactic

Anaphylactoid reactions resemble anaphylactic reactions. However, anaphylactoid reactions, unlike anaphylactic reactions, may occur after the first exposure to a substance. For example, anaphylactoid reactions may occur after the first injection of certain drugs, such as polymyxin, pentamidine Some Trade Names NEBUPENT PENTAM 300 , opioids, or the radiopaque dyes sometimes used with x-ray procedures. Also, anaphylactoid reactions are not allergic reactions because immunoglobulin E (IgE), the class of antibodies involved in allergic reactions, does not cause them. Rather, the reaction is caused directly by the substance.

Aspirin Some Trade Names ECOTRIN ASPERGUM and other nonsteroidal anti-inflammatory drugs (NSAIDs) can cause anaphylactoid reactions in some people, particularly those with year-round allergic rhinitis and nasal polyps. Dyes that can be seen on x-ray (radiopaque dyes) are a common cause. Other triggers include blood transfusions and exercise.

If possible, doctors avoid using radiopaque dyes in people who have anaphylactoid reactions to such dyes. However, some disorders cannot be diagnosed without dyes. In such cases, doctors use dyes that are less likely to cause reactions. In addition, drugs that block anaphylactoid reactions, such as prednisone Some Trade Names DELTASONE METICORTEN , diphenhydramine Some Trade Names BENADRYL NYTOL SOMINEX , or ephedrine, are usually given before the dye is injected.


A mastocytoma usually disappears spontaneously. Itching may be treated with antihistamines. For children, no other treatment is needed. If adults have itching and rashes, ultraviolet light and corticosteroid creams may be applied to the skin.

Systemic mastocytosis cannot be cured, but symptoms can be controlled with antihistamines and histamine-2 (H 2 ) blockers (which reduce acid production in the stomach—see Peptic Disorders: Drugs Used to Treat Peptic Disorders ). Cromolyn Some Trade Names CROLOM INTAL NASALCROM given by mouth can relieve digestive problems and bone pain. Aspirin Some Trade Names ECOTRIN ASPERGUM can relieve flushing but may make other symptoms worse. Children are not given aspirin Some Trade Names ECOTRIN ASPERGUM because Reye's syndrome is a risk.

If systemic mastocytosis is aggressive, interferon-alpha, injected under the skin once a week, may reduce the disorder's effects on bone marrow. Corticosteroids (such as prednisone Some Trade Names DELTASONE METICORTEN ), taken by mouth, may also be used but only for a short time. When taken by mouth for more than 3 to 4 weeks, they can have many, sometimes serious side effects.

If many mast cells accumulate in the spleen, the spleen may be removed. If leukemia develops, chemotherapy drugs (such as daunomycin, etoposide, and mercaptopurine Some Trade Names PURINETHOL ) may help.

A self-injecting syringe of epinephrine Some Trade Names ADRENALIN should always be carried for prompt emergency treatment of anaphylactic reactions.

Last full review/revision September 2008 by Peter J. Delves, PhD

Source: The Merck Manual Home Edition