Mast Cell Activation Syndrome: Symptoms and Treatment

What is mast cell activation syndrome (MCAS)?

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

When you’re exposed to allergens or other substances like medications and insect venom, your mast cells react by releasing substances called mediators. Mediators cause symptoms of an allergic reaction, including itching, mucus, and inflammation.

If you have MCAS, your mast cells release mediators too frequently and too often. It’s different from mastocytosis, another mast cell disorder that happens when your body produces too many mast cells in one or more organs within your body.

What are the symptoms?

The release of too many 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 are 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, your symptoms may include a rapid drop in blood pressure, weak pulse, and narrowed airways in your lungs, making it difficult to breathe. This life-threatening condition is called anaphylactic shock and requires emergency treatment.


There’s no one-size-fits-all diet for MCAS. This is because different foods may trigger symptoms in different people.

Low-histamine diets may help some people manage symptoms of MCAS, though more research is needed. This diet limits foods generally thought to be high in the chemical histamine, which mast cells release when they’re activated. These foods include:

  • hard cheese
  • fish
  • spinach
  • sausage
  • alcohol

A low-FODMAP diet, which eliminates food containing certain kinds of sugars, may also be beneficial for MCAS. 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 reduced participants’ levels of histamine. This suggests that the diet may affect mast cell activity. The diet involves avoiding high-FODMAP foods like:

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

Since you may respond to certain foods differently than other people, it’s important to talk with your doctor or dietitian to find a nutritious diet that will work best for you.

What causes it?

Researchers aren’t sure what causes MCAS. Some studies suggest a genetic component to MCAS, but more research is needed.

It can also be hard to figure out what may trigger MCAS episodes. Common triggers include:

  • allergic-type triggers, such as insect bites or certain foods
  • drug-induced triggers, such as antibiotics, ibuprofen, and opiate pain relievers
  • stress- related triggers, such as anxiety, pain, rapid temperature changes, exercise, being overly tired, or an infection
  • smells, such as perfume or smoke

If your doctor can’t determine a trigger, the condition is called idiopathic MCAS.

How is it treated?

MCAS doesn’t have a cure, but there are ways to manage symptoms. Treating your symptoms can also help you find triggers that cause your MCAS episodes.

Treatments include:

  • H1 or H2 antihistamines. These block the effects of histamines, which are one of the primary mediators that mast cells release. Histamine type 1 receptor blockers include diphenhydramine and loratadine and can help with symptoms like itching and stomach pain. Histamine type 2 receptor blockers include ranitidine and famotidine, which can treat stomach 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. Medications like 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 injection of epinephrine. This can be done at a hospital or with an auto injector (EpiPen). If you often experience severe symptoms, consider wearing a medical ID bracelet until you figure out your triggers.

How is it diagnosed?

MCAS can be challenging to diagnose because it has similar symptoms to many other conditions. There are also disagreements about criteria for diagnosis. As a result, research from 2020 notes that the occurrence of MCAS may be anywhere from rare to affecting as much as 17% of the general population.

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.
  • Taking medications that block the effects or release of mast cell mediators reduces or resolves your symptoms.
  • Blood or urine tests taken during an episode show higher levels of markers for mediators than when you aren’t having an episode.

Before making a diagnosis, your 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.

Mast cell activation syndrome vs. mastocytosis

Unlike MCAS, which features a standard amount of mast cells that release mediators too frequently, mastocytosis occurs when your body produces too many mast cells. These cells can continue growing and tend to be overly sensitive to activation and releasing mediators.

Since there are more mast cells, they release a higher amount of mediators, causing an allergic reaction and sometimes anaphylaxis. These symptoms, along with their treatments, are similar to those in 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 also 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.

What’s the outlook?

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

While the cause of MCAS is still unclear, proper diagnosis and treatment can help you manage your symptoms.

Once you figure out your MCAS triggers, you may also be able to avoid them to reduce your number of episodes.

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