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Systemic mastocytosis (pronounced mass-toe-sigh-TOE-sis), or SM for short, is a rare disease estimated to occur in approximately 32,000 people in the U.S.1 It is caused by an overproduction of abnormal mast cells (a type of white blood cell).


SM is the buildup of abnormal mast cells in different parts of the body such as the skin, liver, spleen, bone marrow, and digestive tract that can cause a variety of symptoms such as rashes, bone pain, and diarrhea. In advanced cases, mast cells can also cause organ damage. While these symptoms can often be associated with other conditions, the combination of multiple symptoms in different parts of the body is common among people with SM. In approximately 95% of cases, SM occurs due to a genetic mutation in the gene called KIT D816V.

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Symptoms Throughout the Body

People with SM report having multiple seemingly unconnected symptoms in different parts of their body—from rashes to GI issues to anaphylaxis and more.

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Tricky Triggers

For people with SM, the things that may cause or trigger symptoms can vary widely. From sensitivity to temperature variations to the development of new allergies, triggers may be unpredictable and may change how people live their lives.

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A Complicated Condition

Some symptoms of SM may seem like common intolerances, or new allergies (for example, developing sudden additional allergies to milk or shellfish, etc.). People with SM sometimes manage their condition by following a strict diet and routinely avoiding triggers at work or in their social lives.

SyMptoms of SM

People with SM may experience different symptoms that may be unexplained, unpredictable, recurring, or severe. Here are some examples:

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Skin Rashes

Hives, rashes that look like “spots,” flushing that lasts more than a few minutes.

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Gut Issues

Diarrhea, nausea, heartburn, abdominal cramps, sensitivities to food.

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Severe or whole-body allergic reactions, including anaphylaxis or bone pain.

The Many Symptoms of SM

Learning more about the many symptoms associated with SM may lead to more informed questions and a more productive discussion with your doctor.


Types of SM

There are two forms of SM: Non-Advanced SM (Non-AdvSM) and Advanced SM (AdvSM). Non-Advanced SM is more common and has two subtypes, whereas Advanced SM is less common and has three subtypes.

Non-Advanced SM

Indolent systemic mastocytosis (ISM) is believed to be the most common form of Non-Advanced SM. ISM may progress to SSM or AdvSM in approximately 5% of cases.2

Smoldering systemic mastocytosis (SSM) has more significant symptoms and can sometimes lead to organ problems. SSM may progress to AdvSM in approximately 9% of cases.2

Advanced SM

SM with associated hematologic neoplasm (SM-AHN) where an additional blood disorder may be present.

Aggressive systemic mastocytosis (ASM), which is associated with progressive organ damage.

Mast cell leukemia (MCL), which is an extremely rare and aggressive form of SM that affects the bone marrow and blood.


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You may have questions. Here are some answers:

What is SM?

Systemic mastocytosis (SM) is a condition where abnormal mast cells (a type of white blood cell) accumulate in different parts of the body such as the skin, heart, digestive track, bone marrow, and other organs. These mast cells release substances, known as mediators, that may cause allergic-like symptoms in these different parts of the body.

What causes SM?

SM is caused by the activation of the KIT receptor protein, which drives the overproduction of abnormal mast cells and their buildup in a variety of organs. This buildup can result in symptoms such as rashes, diarrhea, anaphylaxis, heart palpitations, dizziness, and others. In approximately 95% of SM cases, the overactivation of this protein is associated with a mutation in the KIT gene called KIT D816V.

How many people have SM?

SM is a rare disease and the true incidence rates are unknown, though it is estimated that approximately 32,000 people in the U.S. may be living with SM (or 0.01% of the population).1

How is SM diagnosed?

Along with gathering a full medical history, there are several tests your doctor may order to evaluate you for SM. See some examples below, and please keep in mind that this is not an exhaustive list:

  • A tryptase test measures the amount of tryptase (an enzyme released by mast cells) in your blood.
  • Genetic tests may include KIT D816V, which is a blood test that measures the genetic mutation (KIT D816V) that is a genetic marker in approximately 95% of SM cases.
  • Other tests may include biopsies (skin, bone marrow, liver, spleen, and/or other organs).
What is serum tryptase?

Serum tryptase is useful to evaluate for mast cell activation. Tryptase is a substance that is released from mast cells when the mast cells are stimulated by triggers. The serum tryptase level is used as a diagnostic marker in systemic mastocytosis and may reflect the burden of neoplastic mast cells.

What is a KIT D816V test?

A KIT D816V blood test may be helpful to aid in the diagnosis of patients who are being evaluated for SM. Since the mutation is found in approximately 95% of SM cases, measurement of this genetic marker can help evaluate for SM. There are different types of KIT tests available through several labs, but high-sensitivity tests are more accurate.

What is a biopsy?

A biopsy is a procedure to remove a piece of tissue or a sample of cells from your body that is then analyzed in a laboratory.

What types of doctors usually treat SM?

The management of SM may require doctors across many specialties, such as allergists/ immunologists, dermatologists, gastroenterologists, or hematologists/oncologists, depending on the symptoms present and type of SM.

What triggers the symptoms of SM?

There are many triggers for SM symptoms. Some of those triggers include medications, physical conditions like heat, cold, pressure, stress (emotional, physical, environmental), alcohol, infections, and vibration, among others.

Is SM contagious?

No, SM is not contagious. It is caused by abnormal mast cells that accumulate in different parts of the body.

Is SM hereditary?

SM is not thought to be a condition that is passed down through families. The mutation, KIT D816V, that causes SM in approximately 95% of cases is thought to be spontaneous, rather than inherited.

Does SM get worse over time?

SM is a chronic (life-long) condition with symptoms that vary greatly from person to person. Advanced forms of SM (AdvSM) can be associated with progressive (worsening) organ damage. There is approximately a 5% chance ISM may progress to SSM or AdvSM and approximately a 9% chance SSM may progress to AdvSM.2

What is the difference between Non-Advanced and Advanced SM?

People living with Non-Advanced SM can experience symptoms such as skin rashes and gut symptoms. People with Advanced SM may experience more severe consequences of mast cell accumulation and activation across the body. This may include anemia, low platelet counts, fluid buildup, bone fractures, enlarged spleen and lymph nodes and, in some cases, organ failure.

Is SM an autoimmune disease?

No, SM is a condition where abnormal immune cells (mast cells) build up in different parts of the body. It is not an autoimmune disease (where the body attacks its own healthy tissues).

What is it like to live with SM?

People living with SM may experience a variety of symptoms in different parts of the body. The unpredictable and continual nature of these symptoms can take a toll on people with SM. It’s not uncommon for people to report living with SM as disabling.

What do mast cells normally do?

Mast cells are a type of white blood cell. They’re known for their role in allergic reactions—symptoms like rashes, diarrhea, and anaphylaxis. They also play an important function in wound healing, making new blood vessels, immune tolerance, and defense against bacteria and parasites.

How is SM different from mast cell activation syndrome (MCAS)?

Mast Cell Activation Syndrome (MCAS) is a condition where mast cells are activated, which causes symptoms of anaphylaxis by releasing high levels of mast cell mediators. SM, on the other hand, occurs when the body produces too many mast cells. The presence of KIT D816V on a bone marrow biopsy may be suggestive of a diagnosis of SM and not MCAS.

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References: 1: Data on file. Blueprint Medicines Corporation, Cambridge, MA 2022. 2: Trizuljak J, et al. Allergy. 2020;75(8):1927-1938.