What is mastocytosis?

Mastocytosis (say "mass-toe-sigh-toe-sis") is the abnormal growth of mast cells in the body (usually the skin). Mast cells are part of the immune system. 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. It is usually mild in children, and they often outgrow it.

What are mast cells?

Mast cells are a kind of cell made by your bone marrow. They're part of your immune system, which helps you fight off infections. There are more of these cells in the skin, the lungs and the intestines than in other parts of your body. Mast cells make a chemical called histamine. Normally, this chemical serves as a kind of alarm to let the immune system know that an infection is attacking part of the body. Histamine can cause swelling, itching and redness when your body reacts to something like an insect bite or a bee sting (called an allergic reaction).

What causes mastocytosis?

Doctors don't know exactly why some people have too many mast cells. Some things trigger the release of histamine from mast cells and also cause the symptoms of mastocytosis. Symptoms may be triggered by cold or heat, certain medicines, emotional stress and insect bites. However, the triggers aren't the same in every person.

What are the symptoms of mastocytosis?

Possible symptoms of mastocytosis

  • Red, itchy rash
  • Rash that looks like freckles
  • Hives
  • One large lump on the skin
  • Diarrhea
  • Stomach pain
  • Fainting
  • Difficulty breathing
The symptoms are different, depending on where the extra mast cells are. If there are too many mast cells in your skin, you might have a red and itchy rash. You could get hives or have a rash that looks like freckles. If you rub the rash, it may get red and swollen. Sometimes the mast cells collect at one spot in your skin and cause one large lump.

If the mast cells are in your stomach and intestines, you might have diarrhea and stomach pain. However, this is a more rare form of mastocytosis.

In some people, the extra mast cells cause a serious reaction, similar to a bad allergy reaction. Their blood pressure may suddenly drop to a low level, causing them to faint. They may have trouble breathing. This reaction can cause death if treatment isn't started right away. This kind of serious reaction is also very rare.

How does my doctor know I have mastocytosis?

The symptoms of mastocytosis can be similar to the symptoms of many other health problems. Your doctor may do a skin biopsy to find out what is causing your symptoms. To do a biopsy, your doctor removes a small piece of skin and puts it under a microscope to look for extra mast cells. When an adult gets mastocytosis, a bone marrow biopsy may be needed to look for other blood diseases that might come along with the mastocytosis.

If you don't have a rash but you have other symptoms, such as diarrhea, your doctor may do a blood test or a urine test.

Can mastocytosis be treated?

Yes. Treatment can stop your mast cells from releasing histamine. It can also keep the histamine from causing problems.

Antihistamines (which are often used to treat allergies) are helpful. If a rash bothers you, your doctor may suggest that you be treated with ultraviolet light. If you have diarrhea, an oral version (taken by mouth) of a medicine called cromolyn (brand name: Gastrocrom) may help relieve digestive problems.

The best treatment for mastocytosis may be to stay away from the things that seem to trigger your symptoms. It may help if you and your doctor list all the things that cause you to experience symptoms. Did your symptoms start after you were exposed to heat or cold? Felt stressed? Took a certain medicine? Got an insect bite? Share your list of triggers with your doctor.

Because mastocytosis can cause a severe allergic reaction in some people, it's a good idea to keep an emergency kit with you at all times so you can give yourself medicine to stop a dangerous reaction.

More Information

For more information talk to your doctor.


Written by editorial staff.

Cutaneous and Systemic Manifestations of Mastocytosis by WA Alto, M.D., M.P.H., and L Clarcq, D.O. (American Family Physician June 1, 1999,

Reviewed/Updated: 07/08
Created: 06/99