Systematic mastocytosis is a disorder in which immune cells known as mast cells build up in various internal organs within the body.

Mast cells are normally produced in the bone marrow and found in the surfaces of the body in close contact with the outside world, such as the skin or intestines.

If you have allergies and are exposed to an allergen, your immune system senses a foreign invader. This triggers mast cells to release an inflammatory substance called histamine, causing an allergic reaction.

When these cells build up in parts of the body where they shouldn’t, it’s known as mastocytosis. There are two types of mastocytosis:

  • Cutaneous. Mast cells build up in the skin only.
  • Systemic. Mast cells accumulate in internal organs, such as:
    • bone marrow
    • small intestines
    • liver
    • spleen

Mastocytosis is rare, affecting about 1 out of every 10,000 to 20,000 people. Although children and adults can both experience mastocytosis, systemic mastocytosis is most common in adults.

Indolent systemic mastocytosis is the most common form of systemic mastocytosis, which starts out relatively mild and slowly worsens over time.

Systemic mastocytosis is typically caused by mutations in the KIT gene, which encodes a protein that helps control cell growth and division. When this gene is mutated, it can cause the uncontrolled production of mast cells, which then accumulate in parts of the body where they aren’t needed.

One of the first signs of systemic mastocytosis may be the presence of an itchy rash that occurs when mast cells build up within the skin. This rash may present in a spotty fashion that looks like freckles.

When the skin is irritated, this rash can develop into hives. This is often caused by rubbing or scratching the rash, but some foods and medications can trigger the formation of hives as well.

If you have systemic mastocytosis, it’s important to monitor your triggers and discuss with your doctor whether you should avoid certain foods or medications.

Because systemic mastocytosis affects so many different organs in the body, people may experience symptoms elsewhere than the skin.

According to the National Institutes of Health, the most common symptoms of systemic mastocytosis are:

  • headache
  • abdominal or stomach pain
  • nausea or vomiting

Intestinal malabsorption is also common, which means that the body has trouble absorbing fluid and nutrients from food. Other common symptoms include:

  • diarrhea
  • muscle or bone pain
  • anemia
  • impaired temperature sensation

When mast cells build up, you may also experience symptoms similar to an allergic response, including flushing of the skin, low blood pressure, and shortness of breath. In severe cases, this may cause fainting and even anaphylaxis (a severe allergic reaction).

There’s no cure for systemic mastocytosis, so treatment mainly involves relieving the effects of mast cell overgrowth and avoiding dietary and environmental triggers.

Since mast cells are producers of histamine, antihistamines are usually a component of the treatment plan. These can help prevent many of the skin and gastrointestinal symptoms. Mast cell stabilizers, such as cromolyn sodium, can also prevent the production of histamine.

The goal of most other treatment options is to manage the effects of systemic mastocytosis. Treatments may include:

  • proton pump inhibitors for gastrointestinal symptoms
  • oral steroids to improve nutrient uptake
  • steroid cream to relieve skin inflammation
  • epinephrine for severe allergic reactions

Although aggressive forms of systematic mastocytosis are rare, they can be very serious. In aggressive systemic mastocytosis, the mast cells progressively accumulate in the organs, which can impair function and eventually lead to organ failure.

In even rarer cases, systemic mastocytosis can cause mast cell leukemia, an aggressive cancer characterized by the accumulation of mast cells within the blood or bone marrow.

In 2017, the Food and Drug Administration approved a medication called midostaurin (Rydapt) for the treatment of aggressive forms of systemic mastocytosis, including mast cell leukemia. In a phase 2 clinical study, 60 percent of people with advanced systemic mastocytosis who were treated with midostaurin experienced at least some improvement in their symptoms.

A rash may be the first symptom that leads a healthcare professional to suspect systemic mastocytosis, especially if it’s associated with other characteristic signs and symptoms of the condition.

To confirm if these symptoms are caused by systemic mastocytosis, your doctor will order additional testing, which may include:

  • a bone marrow biopsy
  • bloodwork and a urinalysis, to check for mast cell markers
  • imaging, to determine what organs may be involved
  • genetic testing

If other organs are affected, additional biopsies may be ordered.

For people with indolent systemic mastocytosis, the outlook is generally good. A study that followed 145 people with the condition found progression was usually slow and life expectancy was unchanged.

More aggressive forms of the disease in which multiple organ systems are affected are generally associated with a poorer outlook. However, people with the most aggressive forms of mastocytosis often survive months or a few years after diagnosis.

In the midostaurin study, people with mast cell leukemia survived for a median of 9.4 months on treatment.

Even though systemic mastocytosis is caused by a genetic mutation, it’s generally not an inherited disease.

The mutations that cause the condition are typically new and occur after conception. As a result, they only occur in certain cells of the body. Unless these mutations occur in the germ cells (eggs or sperm), which is very rare, they’re not passed on to future generations.

Because systemic mastocytosis is so rare, it’s important to work with a specialist who is familiar with the condition.

Your healthcare team may be able to connect you with experts in your area. This may involve a team of allergists, immunologists, or a hematologist.

The American Academy of Allergy, Asthma, & Immunology also provides an online database to find allergists and immunologists near you.

Systemic mastocytosis is caused by the accumulation of mast cells in various organs throughout the body, which can cause, among other symptoms, skin conditions, gastrointestinal symptoms, and allergic reactions.

For most people, the outlook is very good, and symptoms can be managed using a combination of antihistamines and other medications.

If you think you may have systemic mastocytosis, talk with a healthcare professional to see if your symptoms should be evaluated by a specialist.