Systemic mastocytosis is a rare blood disorder where the body produces too many abnormal mast cells that accumulate in various organs, causing a wide range of symptoms that can affect the skin, digestive system, bones, and other parts of the body.
Understanding Systemic Mastocytosis
Systemic mastocytosis occurs when your body makes abnormal mast cells, which are a type of white blood cell that plays an important role in your immune system. Mast cells normally help protect you from disease by releasing chemicals like histamine when they detect threats like allergens or infections. In healthy people, mast cells stop releasing histamine once their job is done. However, in systemic mastocytosis, these cells multiply out of control and continue releasing histamine and other chemical mediators long after they should stop.[1][2]
This chemical overload creates a continuous allergic response throughout the body. The abnormal mast cells can build up in many different tissues and organs, including the skin, bone marrow, digestive tract, liver, spleen, lymph nodes, and sometimes the brain, heart, or lungs. When too many mast cells accumulate in an organ, they can interfere with its normal function and, in severe cases, cause organ damage.[1][2]
How Common Is Systemic Mastocytosis?
Systemic mastocytosis is considered a rare disorder. It affects approximately 13 out of every 100,000 people worldwide, which translates to roughly 32,000 people in the United States. The exact number of cases is difficult to determine because the disease is often misdiagnosed or goes undiagnosed for years.[2][5]
The disease affects males and females equally. Most cases of mastocytosis that are limited to the skin appear during childhood, and about 80 percent of these childhood cases improve or resolve completely by adolescence. In contrast, systemic mastocytosis accounts for more than 95 percent of adult cases and usually persists for a longer time.[4]
Many people with systemic mastocytosis see multiple healthcare providers across different specialties before receiving a correct diagnosis. The median time from first symptoms to diagnosis is about 3 years, though some patients go undiagnosed for up to 9 years. This delay happens because the symptoms can be vague and similar to many other conditions, leading to confusion about what is really causing the problems.[21]
What Causes Systemic Mastocytosis?
The underlying cause of systemic mastocytosis is a genetic change, or mutation, in a gene called KIT. This gene provides instructions for making a protein that controls the growth and activity of mast cells. The KIT protein works like a switch that can be turned on and off to regulate how many mast cells are produced and how long they survive.[3][6]
In approximately 95 percent of people with systemic mastocytosis, there is a specific mutation called KIT D816V. This mutation causes the KIT protein to stay turned on all the time, even when it should be switched off. As a result, signaling pathways that control cell growth become overactive, leading to increased production and accumulation of mast cells throughout the body.[3][5][6]
These mutations are usually somatic, which means they are acquired during a person’s lifetime rather than inherited from parents. The mutation happens spontaneously in a single cell, which then creates identical copies, or clones, of itself. In very rare cases, the KIT mutation can be passed down through families, but in most cases it occurs for no apparent reason.[6][7]
Risk Factors for Developing Systemic Mastocytosis
Because systemic mastocytosis is caused by a genetic mutation that occurs spontaneously, there are no clearly identified lifestyle or environmental risk factors that increase the likelihood of developing the condition. The mutation happens randomly in most cases, and scientists have not yet discovered specific behaviors, exposures, or habits that make someone more likely to develop the disease.[6]
Age appears to play some role in the type of mastocytosis that develops. Cutaneous mastocytosis, which affects only the skin, is more common in children. Systemic mastocytosis, which affects multiple organs, is more common in adults and typically appears after adolescence. Most people with systemic mastocytosis develop signs and symptoms in adulthood, though they can appear at any age.[4][6]
While the disease itself cannot be predicted or prevented based on known risk factors, people who have been diagnosed with systemic mastocytosis should be aware of the triggers that can cause their mast cells to release excessive amounts of mediators, leading to symptom flares. Understanding and avoiding these personal triggers becomes an important part of managing the condition once diagnosed.[1]
Symptoms of Systemic Mastocytosis
The symptoms of systemic mastocytosis vary greatly from person to person and depend on where the abnormal mast cells have accumulated in the body. Some people experience mild symptoms that are more of a nuisance, while others have severe, debilitating symptoms that significantly impact their quality of life. The unpredictable nature of symptoms is one of the most challenging aspects of living with this condition.[1][2]
Common symptoms include flushing of the skin, which appears as redness and warmth that can spread across the face and body. Many people develop hives or itchy skin, and some have a specific skin condition called urticaria pigmentosa, which shows up as raised patches of brownish skin that sting or itch when touched or exposed to changes in temperature. When these skin lesions are rubbed or stroked, they often become red, swollen, and more itchy, a reaction called Darier’s sign.[1][2][7]
Digestive symptoms are common and can include abdominal pain, cramping, diarrhea, nausea, and vomiting. Some people develop stomach ulcers because histamine stimulates the production of stomach acid, which can damage the lining of the stomach and intestines. These gastrointestinal problems can lead to malabsorption of nutrients and weight loss in some cases.[1][8]
Many people with systemic mastocytosis experience bone and muscle pain. The abnormal mast cells can accumulate in bone marrow and affect bone density, leading to osteoporosis (weakening of the bones) and an increased risk of fractures. Some people also develop an enlarged liver or spleen, which can cause discomfort in the abdomen.[2][8]
Neurological and psychological symptoms can occur as well. Some people report problems with concentration, memory difficulties, mood changes, anxiety, or depression. These symptoms may be related to the effects of mast cell mediators on the brain and nervous system. Fatigue is extremely common and can be overwhelming for many patients.[1][6]
People with systemic mastocytosis may also experience episodes where multiple symptoms occur all at once. Healthcare providers often refer to these as mastocytosis attacks or flares. During a flare, a person might suddenly experience flushing, rapid heartbeat, lightheadedness, difficulty breathing, and other symptoms that can be frightening and disabling. After these attacks, people often feel extremely tired and lethargic.[2][8]
Nearly half of all individuals with systemic mastocytosis will experience severe allergic reactions, including anaphylaxis. Anaphylaxis is a medical emergency that can cause a sudden drop in blood pressure, difficulty breathing, swelling of the throat, and loss of consciousness. Without immediate treatment with epinephrine, anaphylaxis can be fatal.[6]
Triggers That Can Worsen Symptoms
Mast cells in people with systemic mastocytosis can be triggered to release their mediators by many different factors. These triggers vary from person to person, and what affects one individual may not affect another. Common triggers include alcohol, spicy foods, certain medications (especially aspirin, nonsteroidal anti-inflammatory drugs, and opioids), insect stings, physical exertion, emotional stress, changes in temperature (both heat and cold), and skin irritation from rubbing or friction.[1][4]
Some people develop new food allergies or sensitivities after being diagnosed with systemic mastocytosis. For example, they might suddenly become unable to tolerate milk, shellfish, or other foods they previously ate without problems. Identifying and avoiding personal triggers is an important strategy for managing symptoms and preventing severe reactions.[5]
Types of Systemic Mastocytosis
Systemic mastocytosis is classified into different subtypes based on how aggressive the disease is, which organs are affected, and what symptoms are present. Understanding the type helps doctors predict how the disease might progress and guides treatment decisions.[2][9]
Indolent systemic mastocytosis is the most common type, accounting for about 90 percent of adult cases. In this form, symptoms usually develop gradually over several years. The abnormal mast cells slowly accumulate in the skin, liver, spleen, bone marrow, and gastrointestinal tract. While symptoms can be bothersome, they are generally mild to moderate and vary from person to person. Most people with indolent systemic mastocytosis have a normal or near-normal life expectancy.[2][7][9]
Smoldering systemic mastocytosis involves more significant accumulation of mast cells, particularly in the liver and spleen, which can become enlarged. This type has more severe symptoms than indolent mastocytosis but is still considered a non-advanced form. Some people with smoldering mastocytosis may eventually progress to more advanced forms of the disease.[2][5]
Systemic mastocytosis with an associated hematologic neoplasm occurs when a person has systemic mastocytosis along with another blood disorder, such as myelodysplastic syndrome or myeloproliferative neoplasms. This combination affects about one-fifth of people with systemic mastocytosis. The presence of a second blood disorder makes the condition more complex and requires treatment for both conditions.[2][9]
Aggressive systemic mastocytosis is a severe form where abnormal mast cells multiply rapidly and cause progressive organ damage. The cells can invade bones, making them weak and prone to fractures. People with aggressive systemic mastocytosis often have impaired function of organs like the liver, spleen, or bone marrow. This type progresses more quickly and is associated with a reduced life expectancy.[2][9]
Mast cell leukemia is an extremely rare and aggressive form where systemic mastocytosis transforms into a type of blood cancer similar to acute myeloid leukemia. In this form, mast cells are found in large numbers in the blood and bone marrow. Mast cell leukemia progresses rapidly and has a very poor prognosis, with most patients surviving only months after diagnosis.[2][9]
Mast cell sarcoma is another very rare form where tumors made of abnormal mast cells attack tissues. Very few people with systemic mastocytosis develop mast cell sarcoma.[2]
Prevention of Systemic Mastocytosis
Because systemic mastocytosis is caused by a spontaneous genetic mutation that occurs during a person’s lifetime, there are no known ways to prevent the disease from developing. The mutation happens randomly, and scientists have not identified specific actions people can take to reduce their risk of developing this condition.[6]
However, once someone has been diagnosed with systemic mastocytosis, there are important steps they can take to prevent serious complications and reduce symptom flares. The most critical preventive measure is avoiding triggers that cause mast cells to release excessive amounts of mediators. Each person needs to identify their own specific triggers through careful observation and, if necessary, with guidance from their healthcare provider.[1]
Carrying an epinephrine auto-injector at all times is essential for preventing death from anaphylaxis. People with systemic mastocytosis should be trained in how to recognize the early signs of a severe allergic reaction and how to use their epinephrine injector. They should also ensure that family members, friends, and coworkers know where the medication is kept and how to help in an emergency.[13]
Some people with systemic mastocytosis who have experienced allergic reactions to insect stings may benefit from venom immunotherapy, which involves receiving gradually increasing doses of the venom to reduce sensitivity. This treatment is highly recommended for those with confirmed Hymenoptera venom allergy, as it can prevent life-threatening reactions to future stings.[15]
Regular monitoring and follow-up with healthcare providers is important for detecting any progression of the disease or development of complications like bone loss. Some people may need to take medications preventively to protect their bones, control stomach acid production, or reduce the risk of allergic reactions.[11]
How Systemic Mastocytosis Affects the Body
To understand how systemic mastocytosis affects the body, it helps to know what mast cells normally do. These cells are produced in the bone marrow, the spongy tissue inside certain bones where new blood cells are made. After being produced, mast cells travel through the bloodstream and settle in tissues throughout the body, particularly in areas that come into contact with the outside world, such as the skin, the lining of the airways and digestive tract, and around blood vessels.[4][7]
Mast cells are filled with granules containing powerful chemical mediators, including histamine, tryptase, leukotrienes, prostaglandins, and various enzymes and growth factors. When mast cells encounter something they recognize as a threat, such as an allergen, bacteria, or other stimulus, they release these chemicals into the surrounding tissue. This process is called degranulation.[4]
In healthy people, this release of mediators causes a controlled inflammatory response that helps fight infection or respond to injury. The blood vessels in the area dilate and become more permeable, allowing immune cells to reach the site. This causes redness, warmth, and swelling. Once the threat is eliminated, the mast cells stop releasing mediators and the inflammation subsides.[4]
In systemic mastocytosis, several abnormal processes occur. First, the body produces far too many mast cells because of the activating mutation in the KIT gene. These excess mast cells accumulate in various organs rather than being distributed normally. Second, the abnormal mast cells are more prone to release their mediators in response to triggers that would not affect normal mast cells. They may also release mediators spontaneously, without any obvious trigger. Third, because there are so many more mast cells present, the total amount of mediators released is much greater than normal.[3][4]
The excessive release of histamine and other mediators creates symptoms similar to those of a severe allergic reaction, even when no allergen is present. Histamine causes blood vessels to expand, which leads to flushing, warmth, and a drop in blood pressure. It stimulates nerve endings, causing itching. In the digestive system, histamine increases the production of stomach acid and stimulates intestinal contractions, leading to cramping and diarrhea. In the airways, it can cause narrowing and difficulty breathing.[4]
The physical presence of large numbers of mast cells in organs can also cause direct damage. In the bone marrow, accumulating mast cells can interfere with the production of normal blood cells, potentially leading to anemia (low red blood cell count), increased bleeding due to low platelet counts, or increased susceptibility to infections due to effects on white blood cells. When mast cells accumulate in bones, they can cause bone pain and contribute to loss of bone density.[2][6]
In the liver and spleen, excessive mast cell accumulation can cause these organs to become enlarged and not function properly. This can lead to fluid buildup in the abdomen, a condition called ascites. In severe cases, mast cell infiltration can cause progressive damage to multiple organs, leading to organ failure.[6][9]
The chemical mediators released by mast cells can also affect the brain and nervous system, which may explain the cognitive difficulties, mood changes, and psychological symptoms that many people with systemic mastocytosis experience. The exact mechanisms are still being studied, but it appears that the chronic inflammatory state created by excess mediators can have wide-ranging effects on brain function.[6]
One important substance released by mast cells is tryptase, an enzyme that is abundant in mast cell granules. Because tryptase is released almost exclusively by mast cells, measuring the level of tryptase in the blood has become an important diagnostic tool. People with systemic mastocytosis typically have elevated baseline tryptase levels, and the levels spike even higher during symptom flares or allergic reactions.[4]
