Systemic mastocytosis is a rare blood disorder in which abnormal mast cells build up in organs throughout the body, most commonly the bone marrow, skin, liver, spleen, and gastrointestinal tract. It affects roughly 11 people per 100,000, with about 0.56 new cases per 100,000 diagnosed each year. The disease ranges from a mild, manageable condition to an aggressive form that can damage organs.
Mast cells are immune cells that normally help fight infections and trigger allergic responses. In systemic mastocytosis, a genetic mutation causes these cells to multiply out of control and release excessive amounts of histamine and other inflammatory chemicals into the body.
What Causes Mast Cells to Multiply
More than 90% of people with systemic mastocytosis carry a specific mutation in a gene called KIT. Under normal conditions, mast cells only grow and survive when they receive a signal from a protein called stem cell factor. The KIT mutation essentially flips a switch that keeps the growth signal permanently “on,” so mast cells multiply and survive without that external signal. This leads to a steady, uncontrolled accumulation of mast cells in tissues where they don’t belong.
The mutation is somatic, meaning it develops during a person’s lifetime rather than being inherited from a parent. It is not something you can pass on to your children in most cases.
Symptoms Across the Body
The symptoms of systemic mastocytosis come from two sources: the chemicals that overactive mast cells release, and the physical crowding of mast cells in organs. The chemical-driven symptoms mimic severe allergic reactions and can flare unpredictably.
On the skin, flushing, itching, and hives are common. Many people experience gastrointestinal problems like belly pain, diarrhea, nausea, and vomiting. A fast or irregular heartbeat, dizziness, and fainting can occur. Some people deal with shortness of breath or wheezing. Bone and muscle pain are frequent complaints, as is severe fatigue. The disease also affects mood and cognition: depression, anxiety, irritability, and difficulty concentrating are well-documented symptoms that often go unrecognized.
In more advanced forms, the liver, spleen, or lymph nodes may become noticeably enlarged as mast cells infiltrate these organs.
Common Triggers for Flares
People with systemic mastocytosis learn to identify and avoid situations that cause their mast cells to suddenly release their chemical contents, a process called degranulation. The most common triggers include insect stings, sudden temperature changes, emotional stress, physical exertion, alcohol, skin irritation, and certain medications. Triggers vary from person to person, and some flares happen without an obvious cause. Severe degranulation episodes can look like anaphylaxis, with a dangerous drop in blood pressure, and can be life-threatening.
How It Is Diagnosed
Diagnosis requires a bone marrow biopsy as the central test. Pathologists look for clusters of at least 15 mast cells grouped together in the bone marrow sample. Finding these dense clusters satisfies the major diagnostic criterion. Beyond the biopsy, several supporting findings (called minor criteria) help confirm the diagnosis:
- Mast cell shape: At least 25% of the mast cells in the bone marrow appear spindle-shaped or otherwise abnormal under the microscope, rather than round.
- KIT mutation: Genetic testing of the bone marrow detects the activating KIT mutation.
- Serum tryptase level: A blood test measuring tryptase, a protein released by mast cells, comes back above 20 ng/mL. This is a simple, non-invasive clue that often raises the initial suspicion.
A diagnosis requires either the major criterion plus one minor criterion, or three minor criteria together. Because the symptoms overlap with allergies, irritable bowel syndrome, and anxiety disorders, many people go years before receiving the correct diagnosis.
Types and How They Differ
The World Health Organization classifies systemic mastocytosis into six subtypes, split into two broad categories: non-advanced and advanced.
Non-Advanced Forms
Most people fall into this group. Bone marrow mastocytosis is the mildest form, where mast cell buildup is limited to the bone marrow with minimal symptoms. Indolent systemic mastocytosis (ISM) is the most common subtype overall. People with ISM have symptoms from mast cell chemicals but no organ damage. Smoldering systemic mastocytosis (SSM) involves a higher burden of mast cells and more abnormal lab values, but still no organ damage. These non-advanced forms follow a slow, stable course. In a 15-year study, people with ISM had an overall survival rate of about 84%.
Advanced Forms
Advanced systemic mastocytosis includes three subtypes. Aggressive systemic mastocytosis (ASM) involves organ damage from mast cell infiltration, such as liver dysfunction, dangerous drops in blood counts, or bone deterioration. Systemic mastocytosis with an associated blood cancer (SM-AHN) means a second, separate blood disorder exists alongside the mastocytosis. Mast cell leukemia (MCL) is the rarest and most severe form, with very high numbers of mast cells circulating in the blood. The prognosis for advanced disease is significantly worse: median survival is around 5 years, compared to the much longer life expectancy seen in indolent disease.
Treatment for Non-Advanced Disease
For the majority of people with indolent or smoldering mastocytosis, treatment focuses on controlling symptoms. Antihistamines are the backbone, often taken daily to reduce flushing, itching, and gastrointestinal symptoms. Some people carry injectable epinephrine for emergency use in case of anaphylaxis-like reactions. Avoiding known triggers is a practical, ongoing part of management.
Medications that stabilize mast cell membranes can help reduce the frequency of degranulation episodes. For bone pain or osteoporosis caused by mast cell activity in the bone marrow, treatments to strengthen bones may be recommended. The goal with non-advanced disease is quality of life: reducing flare frequency and severity so daily functioning improves.
Treatment for Advanced Disease
Advanced systemic mastocytosis requires targeted therapy aimed at the underlying KIT mutation. In 2021, the FDA approved avapritinib (Ayvakit) specifically for adults with advanced forms, including ASM, SM-AHN, and mast cell leukemia. This oral medication works by blocking the abnormal KIT protein, directly reducing the number of mast cells and slowing organ damage. It is taken once daily.
Another targeted therapy, midostaurin, was the first drug approved for advanced systemic mastocytosis and works through a similar mechanism of blocking the overactive growth signals driving the disease. The arrival of these targeted treatments has changed outcomes for people with advanced disease, who previously had very limited options. For some patients with advanced disease, stem cell transplant may also be considered.
Living With Systemic Mastocytosis
Because triggers vary widely, managing the disease involves a process of identifying your personal triggers through careful observation. Many people keep symptom diaries to track connections between exposures and flares. Wearing a medical alert bracelet is recommended for those who have experienced anaphylaxis, since emergency responders need to know about the condition.
The unpredictability of flares takes a psychological toll. The depression, anxiety, and cognitive difficulties associated with the disease are partly chemical (driven by mast cell mediators acting on the brain) and partly a response to living with a chronic, poorly understood condition. Finding a specialist familiar with mastocytosis, typically a hematologist or allergist with specific experience, can make a meaningful difference in both treatment and emotional support.