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What is Systemic Mastocytosis?

Systemic mastocytosis (SM) is a rare disease in which immune cells known as mast cells abnormally build up in the bone marrow and other parts of the body. In healthy people, mast cells produce histamine and other so-called “mediators” that help activate the immune system and direct disease-fighting blood cells to where they are needed. In SM, however, the abnormal build-up of mast cells leads to high levels of these mediators, and can cause symptoms that resemble allergies including hives, flushing, shortness of breath and anaphylactic shock. In some patients, mast cells also build up in the organs, impairing organ function and triggering symptoms including abdominal bloating, nausea, pain, fever and fatigue.

Are there different forms of the disease?

Yes. Signs, symptoms and severity of SM can vary widely from person to person. Depending on the nature of your disease, you may be diagnosed with a specific sub-type, or form, of the disease. These sub-types of the disease can be broadly grouped into two categories: indolent and advanced.

Indolent systemic mastocytosis (ISM) is the most common type of SM. People with ISM experience varying degrees of symptoms, but organ function is not impaired and life expectancy is not affected.

Advanced systemic mastocytosis includes aggressive systemic mastocytosis (ASM), SM with associated clonal hematologic non-mast cell disease (SM-AHNMD) and mast cell leukemia (MCL). These forms of the disease are associated with high levels of mast cell build-up in the organs, including the bone marrow, spleen, liver and gastrointestinal tract, that impairs organ function and may lead to shorter life expectancy.

What are the signs and symptoms of the disease?

Signs and symptoms vary from person to person and depending on the specific form of SM. Since histamine and other mediators produced by mast cells are also responsible for allergic reactions, most people with SM experience symptoms that mimic allergies. In many people, one of the first signs of the disease may include “spots” that look like freckles on the skin and can turn into hives if scratched, irritated or exposed to sudden changes in temperature. Other common symptoms include abdominal pain, anaphylactic shock, bloating, pain in the bones, joints and muscles, difficulty breathing, nausea, flushing, headache and fatigue.

Who gets SM?

SM is typically diagnosed in adulthood and does not appear to discriminate by race or gender. SM is very rare, and the exact number of people with disease is not known.

What causes SM?

In almost all patients with SM, a genetic mutation known as KIT D816V is believed to be the root cause of the disease. The genetic mutation is not hereditary, and it is highly unusual for SM to run in families.

How is SM diagnosed?

SM is typically diagnosed by specialists, such as allergists or oncologists. If a physician suspects you have SM based on your symptoms, they may do some initial lab workups, including testing your blood and urine to determine if there are high levels of serum tryptase, a substance produced by mast cells. A definitive diagnosis of SM is reached by taking small tissue samples called biopsies to determine if there is an abnormal build-up of mast cells in the skin, bone marrow and other organs. A genetic test may also be helpful in diagnosing SM since a mutation known as KIT D816V in the KIT gene is believed to be the root cause of the disease in almost all patients with SM.

Because SM is a rare disease, many physicians may not be familiar with it. Symptoms of SM can also resemble other chronic diseases. Together, these factors can contribute to a delay in diagnosis. According to a survey of patients with mastocytosis and other mast cell disorders conducted by The Mastocytosis Society, the median time from onset of symptoms to diagnosis is three years. During that process, patients may see several different physicians and specialists.

How can I find a doctor with experience diagnosing and treating SM?

The Mastocytosis Society provides a list of centers that specialize in SM on its website. You can find the list of centers, along with contact information for those centers, at

Can SM be treated?

Yes, however, treatment options are limited, and currently there is no cure for the disease.

For most patients, the focus of treatment is to control symptoms that are caused by the high levels of histamine and other mast cell-produced mediators. You may be advised by your health care provider to avoid certain dietary and environmental triggers. Anti-allergy medications such as antihistamines are also commonly used to relieve symptoms. Self-injectable epinephrine, often referred to as epi-pens, may be prescribed to treat cases of severe allergic reactions.

For patients with advanced SM, in addition to medications to alleviate symptoms, chemotherapy or steroids may be used to reduce mast cell levels and improve organ function. Only one therapy, imatinib, has been approved for SM by the US Food and Drug Administration, and it is only approved for a small percentage of SM patients with advanced disease who do not have the KIT D816V genetic mutation.

Are there any new treatments being developed?

Yes. Several investigational medicines designed to target the underlying cause of SM are being studied in clinical trials.

Where can I find information about clinical trials of new treatments for SM?

The National Institutes of Health maintains a list of all federally and privately supported clinical trials at For a list of clinical trials for SM, go to the website and enter “Systemic Mastocytosis” into the search field. To narrow the list of clinical trials to those currently enrolling patients, you can check the box “Include only open studies,” which will appear at the top of the search results.

The Mast Cell Connect patient registry is another source of information on research studies and clinical trials for mastocytosis. For more information on the patient registry, please go to