Systemic Mastocytosis

Systemic mastocytosis is a rare blood disorder. It happens when your body makes abnormal mast cells. Mast cells are white blood cells and part of your immune system. They’re first responders when allergens and other intruders attack your body. In systemic mastocytosis, abnormal mast cells multiply, setting up a continuous allergic response that may affect your skin and internal organs.

Healthcare providers can’t cure systemic mastocytosis, but they can treat symptoms and complications the condition causes. Most people with systemic mastocytosis have mild symptoms they can manage with medication and by avoiding activities that trigger the condition. Rarely, systemic mastocytosis symptoms get worse or become cancer. All people with this condition have an increased risk of anaphylaxis. This is a severe allergic reaction that can be life-threatening.

How does this condition affect my body?

You have mast cells throughout your body, mostly in your skin, lungs and the lining of your intestine. Mast cells protect your body from intruders such as bacteria and allergens. Mast cells also help produce stomach acid.

When mast cells detect intruders, they release histamine. Histamine sets off an inflammatory (allergic) response. At the same time, small sacs within each mast cell called mediators release substances to defend against intruders.

Normally, mast cells and mediators stop their work after dealing with intruders. In systemic mastocytosis, you have large numbers of abnormal mast cells continually releasing histamine and other substances. This chemical overload can cause a range of issues.

For example, abnormal mast cells that build up in your bone marrow may affect blood cell production. Likewise, unusually large numbers of mast cells affect organ function. For example, if there are unusually large numbers of mast cells in your stomach, they may produce too much stomach acid and cause ulcers.

Systemic mastocytosis typically affects adults. Another type of mastocytosis — cutaneous mastocytosis — typically affects children.

There are six systemic mastocytosis types. Each type may affect you in different ways. In general, the more abnormal mast cells you have, the greater the chance you’ll have serious health issues. Systemic mastocytosis types include:

• Indolent systemic mastocytosis: This is the most common type of systemic mastocytosis. Indolent systemic mastocytosis symptoms develop over several years. As the number of abnormal mast cells slowly increases, you may notice changes in your skin, liver, spleen and gastrointestinal tract. Most people with indolent systemic mastocytosis manage their symptoms with medication and have normal lifespans.
• Systemic smoldering mastocytosis: Abnormal mast cells multiply in your liver and spleen. Eventually, the cells may cause an enlarged spleen (splenomegaly) or enlarged liver (hepatomegaly).
• Systemic mastocytosis with associated hematologic neoplasm: In this type, people develop blood disorders such as myeloproliferative neoplasms and myelodysplastic syndrome. This type affects about one-fifth of people with systemic mastocytosis.
• Aggressive systemic mastocytosis: This type may affect your bone marrow and bones. As abnormal mast cells multiply in your bone marrow, they interrupt blood cell production. You may develop conditions such as low white blood cell numbers (leukopenia) and low red blood cell numbers (anemia). Abnormal mast cells may also grow into your bones, making them weak and more likely to break.
• Mast cell leukemia: Very rarely, systemic mastocytosis becomes mast cell leukemia. This is a form of acute myeloid leukemia (AML).
• Mast cell sarcoma: This condition involves tumors made of abnormal mast cells that attack tissue. Like mast cell leukemia, very few people with systemic mastocytosis develop mast cell sarcoma.

Is systemic mastocytosis a serious illness?

It can be. While most people don’t have severe types of systemic mastocytosis, they do have an increased risk of anaphylaxis. Some people may go into anaphylactic shock, which is life-threatening.

How common is this condition?

Systemic mastocytosis isn’t common. A 2018 population analysis concluded that 1 in 10,000 people in the U.S. has the condition.