Mast cell activation syndrome (MCAS) is a condition that occurs when the mast cells in your body release too much of a substance that causes allergy-like symptoms.

Mast cells are part of your immune system. They’re found throughout your body, particularly in your bone marrow and around blood vessels.

When people are exposed to allergens, including medications, foods, and insect venom that they’re allergic to, mast cells typically react by releasing chemical mediators. These mediators cause symptoms of an allergic reaction, including itching, mucus, and inflammation.

If you have MCAS, your mast cells release these same mediators too frequently and without exposure to an allergen. Mastocytosis, a different condition than MCAS, happens when your body produces too many mast cells in one or more organs within your body. MCAS can also occur without a known cause.

Keep reading to learn the causes and symptoms of MCAS and how it’s diagnosed and treated.

Researchers aren’t sure what causes some people to experience MCAS. Some studies have been exploring the possibility of a genetic component to MCAS, but more research is needed to fully understand what puts a person at greater risk of developing the condition.

Researchers have identified three variants of MCAS. They include:

  • Primary MCAS: Primary MCAS occurs when a certain mutation, known as the KIT D816V mutation, is found and the mast cells display CD25, with or without a confirmed case of mastocytosis. Mastocytosis occurs when the body produces too many mast cells.
  • Secondary MCAS: Secondary MCAS occurs as an indirect result of another immunologic condition, IgE-mediated allergen (food or environmental allergy), or hypersensitivity to another trigger.
  • Idiopathic MCAS: Idiopathic means that the cause of MCAS can’t be determined. Unlike primary MCAS, it’s not the result of a cloned cell. And unlike secondary MCAS, a doctor or healthcare professional can’t determine an underlying trigger for the MCAS.

The release of too many mast cell mediators can impact almost every part of your body.

The primary affected areas typically include your skin, nervous system, heart, and gastrointestinal tract. The number of mediators released can cause symptoms that range from mild to life threatening.

Symptoms may include:

  • skin: itching, flushing, hives, sweating, swelling, rash
  • eyes: irritation, itching, watering
  • nose: itching, running
  • mouth and throat: itching, swelling in your tongue or lips, swelling in your throat
  • lungs: trouble breathing, wheezing
  • heart and blood vessels: low blood pressure, rapid heart rate
  • stomach and intestines: cramping, nausea, diarrhea, abdominal pain
  • nervous system: headache, confusion, fatigue

In severe cases, you may experience anaphylactic shock. This condition requires emergency treatment. Symptoms can include:

  • a rapid drop in blood pressure
  • lightheadedness
  • weak pulse
  • trouble breathing or quick and shallow breathing
  • confusion
  • loss of consciousness

If you or someone else experiences symptoms of anaphylactic shock, call 911 or your local emergency services.

Both MCAS and mastocytosis are classified as mast cell diseases (MCD),

MCAS occurs when the mast cells in your body release too much of the mediator substance that causes allergy-like symptoms. Mastocytosis occurs when your body produces too many mast cells. These cells can continue growing and tend to be overly sensitive to activation and mediator release.

Because there are more mast cells, they release a higher amount of mediators, causing an allergic-like reaction and sometimes anaphylaxis. These symptoms are similar to those in idiopathic MCAS.

Mastocytosis may be cutaneous — in which the higher numbers of mast cells are only present in the skin — or systemic, in which the mast cells are in other organs.

Cutaneous mastocytosis often causes skin lesions. Systemic mastocytosis may lead to a larger liver or spleen, or reduced organ function. Mast cell leukemia is a rare form of mastocytosis that may develop over time.

Skin or bone marrow biopsies may be used to look for elevated numbers of mast cells. The presence of the mutation called KIT D816V causes the ongoing growth of mast cells, along with their activation, and can also indicate mastocytosis.

An American Academy of Allergy, Asthma & Immunology work group report proposed the following criteria for diagnosing MCAS:

  • You have recurrent, severe symptoms (often anaphylaxis) that affect at least two organs.
  • Tests show elevated levels of one or more mast cell mediators
  • Blood or urine tests taken during an episode show higher levels of markers for mediators or their metabolites than when you aren’t having an episode.

Other mediators can also be involved, but they’re not specific to MCAS. Still, an unexpected increase in their levels could indicate MCAS. Markers can include:

  • histamine (plasma, urine)
  • prostaglandin (PG) D2
  • leukotriene (LT) C4
  • histamine metabolites (urine)
  • 24-hour urine PGD2 metabolite
  • 11β-prostaglandin F2α or the LTC4 metabolite (LTE4) level (urine)

Before making a diagnosis, the doctor will likely review your medical history, do an exam, and order blood and urine tests to check for other possible causes of your symptoms. They may also use bone marrow tests to confirm the diagnosis of MCAS rather than mastocytosis.

There’s currently no cure for MCAS, but there are ways to manage the symptoms.

Treatments can include:

  • H1 or H2 antihistamines. These block the effects of histamine, which is one of the primary mediators that mast cells release. Histamine type 1 receptor blockers include cetirizine and loratadine and can help with symptoms, such as itching and stomach pain. Histamine type 2 receptor blockers include ranitidine and famotidine, which can treat abdomen pain and nausea.
  • Aspirin. This may decrease flushing.
  • Mast cell stabilizers. Omalizumab may help prevent the release of mediators from mast cells, resulting in fewer episodes of anaphylaxis.
  • Antileukotrienes. Antileukotrine medications, such as zafirlukast and montelukast, block the effects of leukotrienes, another common type of mediator, to treat wheezing and stomach cramps.
  • Corticosteroids. These should only be used as a last resort for treatment of edema, wheezing, or hives.

If you develop anaphylactic shock or other severe symptoms, you’ll need an epinephrine injection. This can be done at a hospital or with an auto injector (EpiPen). If you use an epinephrine auto injector, you should still visit the emergency room.

If you often experience severe symptoms, consider wearing a medical ID bracelet until you figure out your triggers.

Currently, no studies examine or look directly at the relationship between changes in diet and MCAS.

If a doctor diagnoses you with secondary MCAS, you may find that certain foods trigger your symptoms. You should discuss changes to your diet with a doctor and avoid foods that trigger a reaction.

If you experience symptoms of an allergy after eating a certain food, you may have an allergic reaction rather than MCAS.

Some anecdotal evidence suggests that diet changes may help those with MCAS. However, science and large-scale evidence don’t back up their use for MCAS. A person may find some benefit to trying one of the diets unrelated to MCAS.

Low histamine diet

Anecdotal evidence suggests that low histamine diets may help some people manage symptoms of MCAS, though scientific research doesn’t currently support this. A low histamine diet limits foods generally thought to be high in the chemical histamine, which mast cells release when they’re activated.

Foods that are high in histamine can include:

  • hard cheese
  • fish
  • spinach
  • sausage
  • alcohol

Low FODMAP diet

Some people believe that a low FODMAP diet can help MCAS. However, this isn’t enough scientific evidence to back up this claim. A low FODMAP diet eliminates food that contains certain kinds of sugars. The idea is to restrict certain foods and then reintroduce them to determine which ones may trigger symptoms.

A 2019 study in people with irritable bowel syndrome (a condition in which mast cells may play a role) found that the low FODMAP diet significantly lowered participants’ levels of histamine. This suggests that the diet may affect mast cell activity. The diet involves avoiding high FODMAP foods such as:

  • dairy
  • wheat
  • legumes
  • certain fruits, including apples and peaches
  • certain vegetables, including asparagus and broccoli

It’s important to talk with a doctor or dietitian before making dietary changes in order to ensure you receive the necessary amount of nutrients.

MCAS can cause unexpected allergic-like symptoms that interfere with your daily life.

While the exact causes of MCAS are still unclear, proper diagnosis and treatment can help you manage your symptoms.