What is Mast Cell Activation Syndrome (MCAS)?

Mast cell activation syndrome (MCAS) is a rare disorder in which mast cells in the body become overactive. As a result, they release too many chemicals, such as histamine, impacting multiple body organs. Learn about MCAS symptoms, common triggers, and the risk of anaphylaxis.

At a Glance: Mast Cell Activation Syndrome

  • Overactive cells: MCAS is a condition in which mast cells become overactive. They release chemicals like histamine in response to triggers.
  • Wide-ranging symptoms: Symptoms are broad and impact multiple organ systems. They ften include skin rashes, nausea, shortness of breath, abdominal pain, and rapid heart rate.
  • Anaphylaxis risk: A major concern for people with MCAS is the risk of life-threatening anaphylaxis. This can result from a sudden onset of severe symptoms. Epinephrine is the first-line treatment for anaphylaxis and is essential treatment for severe MCAS symptoms.
  • Common triggers: MCAS can be triggered by temperature changes, allergens (like pollen), certain medications, foods, and alcohol.

Mast cell activation syndrome (MCAS) is a type of mast cell disease or disorder. It occurs when mast cells are too active in your body. MCAS is considered a rare disease.

Everyone has mast cells. They are a type of white blood cell involved in the body’s immune system. They help your body fight off infections and help with the healing process. Mast cells do this by releasing chemical mediators, including histamine.

Normally mast cells release the chemicals when they need to, such as in response to an allergen. But in MCAS, these cells are overactive. They release the chemicals too easily and too often, even when they shouldn’t. This can cause a host of symptoms affecting multiple body organs. People with MCAS may often have severe allergic reactions, or anaphylaxis.

MCAS is sometimes called mast cell activation disease. You may also see it referred to as mast cell activation disorder.

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What Causes Mast Cell Activation Syndrome?

MCAS is mast cell disorder that leads to overactive mast cells. The problem may be linked to the KIT gene, particularly the D816V mutation. Most people with D816V mutation also have another mast cell disorder called systemic mastocytosis.

Many people with MCAS experience symptoms after they are exposed to an allergen, odor or temperature change. Others may not be able to identify a trigger, however.

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Signs and Symptoms of Mast Cell Activation Syndrome

When you encounter an allergen or other trigger, it wakes up your mast cells. This mast cell activity releases chemical mediators. It results in MCAS symptoms that can affect multiple body organs.

MCAS symptoms can vary from person to person. Common symptoms include:

  • Skin: Rash, flushing, and swelling.
  • Stomach: Abdominal pain, diarrhea, nausea, vomiting and heartburn (acid reflux).
  • Heart: Blood pressure changes, rapid heart rate, fainting or dizziness.
  • Breathing: Shortness of breath.

People with MCAS may also experience headaches, brain fog, fatigue, bone pain and weakness in bone marrow. But these symptoms are not caused by mast cell activation.

People with MCAS are at risk for severe allergic reactions, or anaphylaxis. This involves multiple body organs and is a potentially life-threatening condition.

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What are symptoms of anaphylaxis?

Anaphylaxis  is a severe or life-threatening allergic reaction. When there are overactive mast cells in the body, the mast cells release a flood of chemical mediators that can result in anaphylaxis. Symptoms typically involve two or more body systems and may include:

  • Skin: hives, itchiness
  • Mouth: itchiness around the mouth, swelling of lips
  • Throat: swelling of tongue or throat, difficulty swallowing
  • Stomach: abdominal pain, vomiting, diarrhea
  • Breathing: shortness of breath, nasal congestion, wheezing, coughing, chest tightness
  • Heart: blood pressure changes, dizziness, fainting, loss of consciousness
  • Brain: Confusion, feeling of impending doom
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Anaphylaxis is a medical emergency. The first-line treatment for anaphylaxis is  epinephrine . It is important to give epinephrine first and fast. Follow up with care in an emergency department if symptoms are severe, or they worsen or persist. Administer a second dose if symptoms are severe, if they worsen or persist, or if they come back. Then after the second dose, get follow-up care at an emergency department or clinic.

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What Triggers Mast Cells?

Triggers for mast cell activation can vary. Here are some common ones:

  • Heat, cold or temperature change
  • Allergens such as pollen and mold
  • Bug bites or stings
  • Some food and beverages (including alcohol), but not for everyone with MCAS
  • Alcohol
  • Certain drugs, including non-steroidal anti-inflammatory drugs (NSAIDs) and antibiotics
  • Friction
  • Stress
  • Odors or fragrances
  • Exercise
  • Contrast dyes from X-rays

Allergy immunotherapy, or allergy shots for pollen or mold, may also trigger systemic MCAS reactions.

Sometimes there is no trigger found. This is called idiopathic mast cell activation syndrome. “Idiopathic” means there is no known cause.d. This is called idiopathic mast cell activation syndrome. “Idiopathic” means there is no known cause.

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How is Mast Cell Activation Syndrome Diagnosed?

Diagnosing MCAS is a challenge because symptoms can vary widely among patients. MCAS can also mimic other diseases. You may way want to see a specialist such as a board-certified allergist and immunologist for a diagnosis. Other specialists include a gastroenterologist, dermatologist, endocrinologist and hematologist.

A diagnosis typically involves a clinical evaluation, the patient’s medical history, and laboratory tests. If you have chronic allergies and symptoms in other organ systems, this could be a sign of a MCAS.

There are certain criteria people must meet to confirm a MCAS diagnosis:

  • Elevated mast cell mediators (during symptoms). Levels may be twice as high as when no symptoms are present.
  • History of anaphylaxis or severe allergic reactions involving multiple body systems.
  • Symptoms improve with treatment that blocks or slows mast cell activation.

Your doctor will want to conduct blood tests and in some cases urine tests. The doctor will look for high levels of histamine and serum tryptase, a type of protein, in your blood. These are the mast cell mediators that get elevated for MCAS symptoms.

Another MCAS diagnostic test is a biopsy. This involves removing a small piece of skin, bone marrow or the stomach and then examining it under a microscope for signs of mast cells. Tissue biopsies are used to rule out other mast cell disorders.

Your doctor may also recommend bone scans, CT scans and genetic testing to help with the diagnosis.

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What is the Treatment for Mast Cell Activation Syndrome?

There is no cure for MCAS. But it can be managed effectively with medications. It’s also important to know your triggers and avoid them.

MCAS symptoms can affect more than one body organ and lead to anaphylaxis. The first-line treatment for anaphylaxis is epinephrine. All people with MCAS should carry two doses of epinephrine. Products available include an epinephrine nasal spray and auto-injector. If anaphylaxis symptoms occur, give epinephrine first and fast. Do not delay! If symptoms are severe, or they worsen or persist, seek emergency medical treatment at a hospital or clinic.

Medications that can help manage and treat MCAS include:

Antihistamines (H1 blockers)

Antihistamines block the effects of histamine released by mast cells. H1 antihistamines are used to treat allergy symptoms. Many types are available over the counter and by prescription. Second generation antihistamines are often preferred as they cause less drowsiness.

Antacids (H2 blockers)

Antacids are used to reduce stomach acid, often due to acid reflux or GERD caused by mast cell activation. They can also help stomach pain and nausea. These include the over-the-counter ranitidine or famotidine.

Aspirin

Aspirin can be used in certain people with MCAS to reduce symptoms of flushing, brain fog and bone pain. It should only be taken under the supervision of a doctor. In some cases, aspirin can actually worsen symptoms.

Epinephrine

MCAS symptoms can affect more than one body organ. This can lead to anaphylaxis. The first-line treatment for anaphylaxis is epinephrine. All people with MCAS should carry two doses of epinephrine. Products available include an epinephrine nasal spray and auto-injector. If anaphylaxis symptoms occur, give epinephrine first and fast. Do not delay!

If your symptoms improve after one dose of epinephrine and you’re feeling better, you do not need to go the hospital or emergency department. However, you should administer a second dose and seek emergency medical help if…

  • symptoms return or worsen after the first dose of epinephrine;
  • you have severe anaphylaxis;
  • symptoms do not go away promptly or completely after the first dose of epinephrine.

If you do not have epinephrine available, call 911 for an ambulance or go to the hospital or emergency department right away.

Mast cell stabilizers

Mast cell stabilizer medications prevent mast cells from releasing the chemical mediators that cause symptoms. Cromolyn sodium and ketotifen are mast cell stabilizers. Cromolyn sodium is poorly absorbed from the digestive system into the rest of the body when it is taken orally as a pill. So the medication should be used to primarily treat digestive symptoms.

Leukotriene modifiers

These medications reduce the effects of leukotrienes, a type of inflammatory mediator release. They may be useful in some people with mast cell activation syndromes. Montelukast, zafirlukast and zileuton are three leukotriene inhibitors. Monteleukast has a black box warning for increased risk of behavioral and mood-related side effects. Zileuton requires monitoring of liver function by a doctor.

Oral corticosteroids

Oral corticosteroids are used to treat inflammation caused by mast cell activation. They can relieve skin lesions and hives, skin swelling, and breathing problems such as wheezing. Oral corticosteroid pills are given as a short course or burst. Long-term use is not recommended due to potential severe side effects, such as weight gain, diabetes sleep problems, thinning skin, bone weakness and mood swings.

Omalizumab (brand name Xolair®)

Omalizumab is a biologic medication. It is not FDA-approved to treat MCAS, however some doctors may prescribe it “off-label.” Omalizumab is a biologic that blocks the protein antibody immunoglobulin E (IgE) from binding to mast cells. It can reduce mast cell reactivity and sensitivity. This can reduce the risk of severe allergic reactions. Omalizumab is typically administered as an injection every 2-4 weeks. The first three injections must be taken at a doctor’s office. Omalizumab is FDA-approved to treat allergic asthma, chronic hives, chronic rhinosinusitis with nasal polyps, and food allergy.

Questions & Answers (Q&A) about Mast Cell Activation Syndrome (MCAS)

You may have specific questions about MCAS. Here’s a Q&A with some common questions about this syndrome. If there’s another question about MCAS you’d like to see answered, please email the editor.

Mast cell activation syndrome and mastocytosis are disorders involving mast cells. They can cause similar symptoms after the release of mast cell chemical mediators. Both can lead to anaphylaxis.

But these are also two distinct conditions.

MCAS involves mast cells that are overly reactive when they release chemical mediators such as histamine. It is diagnosed when there are severe reactions involving multiple body organs.

Mastocytosis involves the buildup of mast cells. This occurs in body organs and tissues, such as the skin, bone marrow, liver, spleen and the digestive tract. The buildup leads to symptoms and can progress to serious disease.

It is possible for people to have both mastocytosis and mast cell activation disorders, but this is very rare.

While there is no specific “mast cell activation syndrome diet,” some patients find that a low-histamine diet helps manage symptoms. The reason is that a low-histamine diet reduces the external load of histamine the body must process. Common triggers can include fermented foods, aged cheeses, alcohol, and certain processed meats.

MCAS does not affect life expectancy for most people. It is a chronic condition that requires lifelong management. With proper treatment and trigger avoidance, most people with MCAS live a full life.

There is no cure for mast cell activation syndrome. The best way to manage it is to know your triggers and take medication. Avoiding triggers can lead to less frequent symptoms. Medications can help reduce the severity of symptoms.

When someone has idiopathic mast cell activation syndrome, the “idiopathic” means the cause is unknown. This diagnosis is used when a patient meets all the criteria for MCAS (elevated mediators, multi-organ symptoms, and response to treatment). But there is no specific genetic mutation (like KIT D816V) or underlying primary cause can be identified.

Other mast cell disorders include:

  • Mastocytosis, a mast cell disorder involving several types, from mild to severe. 
  • Hereditary alpha tryptasemia (HαT), a genetic trait that can cause frequent and severe mast cell-mediated reactions like anaphylaxis. 
  • Note: HαT has been linked to mastocytosis, MCAS with idiopathic anaphylaxis, and severe reactions to stinging insects.

Mast Cell Disease Resources

These resources can help you prepare for appointments, plan for emergencies, and connect with others who understand life with mast cell disease. 

New to mast cell disease? Start here →

A new diagnosis can bring many questions. You do not need to learn everything at once. The Mast Cell Disease Society (TMS) offers information about mast cell diseases, testing, treatment, and life after diagnosis. 

Visit The Mast Cell Disease Society and choose “New Patients – Start Here” to begin. 

Find a doctor familiar with mast cell diseases 

Finding a healthcare professional who understands your condition can help you take the next step in your care. TMS offers a Find an HCP Resource to help you explore care options. 

Before scheduling, ask whether the doctor treats your specific condition, sees patients in your age group, and requires a referral. You can also check whether the office accepts your insurance. 

Track symptoms and prepare for appointments 

A symptom record can help you explain what happens between doctor visits. TMS offers a symptom tracker and patient guides to help you record symptoms, possible triggers, medicines, and reactions. 

Bring your notes, medicine list, and questions to your appointment. You may want to ask: 

  • What do my test results mean? 
  • What should I do when my symptoms get worse? 
  • How will we know whether my treatment is working? 
  • Which symptoms need emergency care? 

Use this MCD Glossary to look up unfamiliar words. Ask your healthcare team to explain how each term relates to your care. 

Prepare for emergency care 

Planning ahead can make it easier to share important information during an emergency. TMS provides an Emergency Alert Card and an Emergency Room Response Plan through its patient resources page. 

Review these materials with your doctor. Ask your doctor to help create a plan specific to your needs, including when to use prescribed emergency medicine and when to seek emergency help. Share your plan with family members and others who may help during a reaction. 

Connect with others who understand MCD 

Living with mast cell disease can feel isolating. Talking with people who have similar experiences may help you feel understood and give you a place to share everyday concerns. 

TMS offers virtual support groups and online communities. Visit TMS’s Find Support page to explore ways to connect. 

Find information for everyday life about Mast Cell Disease 

You may have questions about daily routines, relationships, or the emotional effects of living with a long-term condition. These Allergy & Asthma Network guides offer more information: 

October 2026
Reviewed by:
Jonathan Lyons, MD, is a board-certified allergist and immunologist at the University of California, San Diego. He is chief of the Translational Allergic Immunopathology Unit in Laboratory of Allergic Diseases at the National Institutes of Allergy and Infectious Diseases (NIAID). Dr. Lyons has received NIAID Merit Wards for his groundbreaking work in characterizing pathways leading to increased mast cell hyperactivity and anaphylaxis in people with hereditary alpha tryptasemia.