Mast Cell Activation Syndrome

Mast cell activation syndrome (MCAS) is when you have unexplained episodes of severe symptoms like swelling, diarrhea, vomiting, flushing and itching. Unlike allergies that happen with a specific exposure, MCAS episodes happen without a clear trigger.

In some cases, mast cell activation can cause anaphylaxis, a severe allergic condition that can be life-threatening. It can cause you to have trouble breathing and drop your blood pressure to dangerously low levels. Call 911 (or your local emergency service number) or go to the nearest emergency room if you’re experiencing severe anaphylaxis.

Mast cells are a type of immune cell responsible for immune reactions. For instance, conditions like allergic rhinitis and asthma, allergic reactions (like anaphylaxis to drugs or food) and mastocytosis all activate mast cells. This causes them to release proteins that give you symptoms that are bothersome at best, and dangerous at worst.

Other common conditions can also cause unexplained symptoms. It’s important to discuss your concerns with your provider. They’ll make sure you get a complete workup with a specialist, like an allergist. An allergist is a type of doctor who can diagnose and manage many types of allergic conditions.

But mast cell activation syndrome is rare. Healthcare providers diagnose mast cell activation syndrome if:

• You have repeated symptoms of possible anaphylaxis without a clear trigger
• You have more than one body system affected at the same time
• Tests show signs of mast cell activation
• Mast cell medications provide relief from your symptoms

Mast cells and their function

Mast cells are a part of your immune system. Their usual function is to help protect your body from invading germs (pathogens — like parasites or viruses) or other harmful substances. They sit in your tissues. When they recognize something they think is harmful, they release histamine and other chemicals to activate your immune system and try to flush out the intruder.

What are the symptoms of mast cell activation syndrome (MCAS)?

MCAS causes repeated episodes of severe symptoms in multiple areas of your body. Symptoms may include:

• Low blood pressure
• Flushing (hot, red or pink skin)
• Itchy skin
• Hives
• Nasal congestion
• Swelling, often of your face, lips, eyes, tongue or throat (angioedema)
• Constipation or diarrhea (sometimes alternating)
• Abdominal (belly) pain
• Vomiting
• Shortness of breath
• Weakness or fainting
• Anaphylaxis (severe allergic reaction, which may include a combination of the above symptoms)

To be considered MCAS, the symptoms must affect two or more body systems. These include your:

• Skin
• Respiratory system (lungs and airways)
• Cardiovascular system (heart and blood vessels)
• Digestive system (GI tract — stomach and intestines)

What does a mast cell flare feel like?

If you’ve ever had an allergic reaction, you know what mast cell activation feels like. But for people with MCAS, episodes are often more severe than typical allergic reactions, and they involve multiple parts of the body. Your skin might get swollen and red, itch or break out in hives. You might get diarrhea or vomit. In extreme cases, the reaction is so severe that it can cause potentially life-threatening symptoms. Call 911 (or your local emergency service number) or go to the nearest emergency room if you’re experiencing severe symptoms.

What causes mast cell activation syndrome?

Your mast cells reacting when they shouldn’t causes MCAS. Mast cells release proteins that cause your symptoms when they think they need to protect you from something harmful that’s entered your body. With allergies, you can identify something specific that causes your body to overreact. In MCAS, there doesn’t seem to be a clear trigger that causes your mast cells to release proteins.

No one’s really sure what causes your mast cells to get sensitive to things they shouldn’t be.

What can trigger it?

While many things can trigger mast cells to activate and release histamine (like food or pollen allergies), MCAS causes severe symptoms that aren’t predictably triggered by anything specific. You might find that you’re more likely to have episodes when you’re experiencing changes in your life or environment, like periods of high stress or shifting weather. If you find that episodes only happen after exposure to something specific, it’s not likely that you have MCAS.

How is mast cell activation syndrome (MCAS) diagnosed?

A healthcare provider might think you have mast cell activation syndrome based on a physical exam, discussion of your symptoms and your medical history. Providers diagnose mast cell activation syndrome if you:

• Have symptoms of mast cell activation
• Respond to treatment for mast cell activation
• Have signs of mast cell activation on testing

MCAS testing

To test for MCAS, your provider may order a blood test to look for a sign of mast cell activation called tryptase. You have to get tested multiple times to check your levels, both when feeling well and when you’re having an episode. Keep in mind that tryptase can be elevated in other conditions, so tryptase levels alone don’t indicate MCAS. Some people naturally have higher-than-expected tryptase levels without symptoms.

Your provider may also do an allergy skin test or allergy blood tests to rule out other causes of your symptoms.

Healthcare providers don’t diagnose mast cell activation syndrome based on a histamine blood test. Elevated histamine levels don’t mean you have MCAS.

How is mast cell activation syndrome (MCAS) treated?

Providers use treatment to both diagnose MCAS and reduce your symptoms. If you don’t improve after treatment with mast cell-targeted medications, you probably don’t have MCAS.

Depending on your symptoms, your provider might prescribe:

• Histamine type 1 receptor blockers (antihistamines): These can treat allergy symptoms and include loratadine, cetirizine and fexofenadine.
• Histamine type 2 receptor blockers (antihistamines that reduce stomach acid): These can treat nausea and stomach pain and include ranitidine or famotidine.
• Aspirin: Aspirin can reduce flushing.
• Leukotriene modifiers: These can help you breathe better and include montelukast and zafirlukast.
• Omalizumab (XOLAIR®): This can reduce anaphylactic episodes.
• Epinephrine: Providers use epinephrine to stop anaphylaxis. You can carry an epinephrine injector (EpiPen®, Auvi-Q®) with you in case you have an anaphylactic episode.
• Mast cell stabilizers: Medications like cromolyn sodium prevent mast cells from releasing mediators (degranulation).
• Corticosteroids: These reduce inflammation and can open up your airways and reduce fluid in your tissues. Due to the risk of side effects, providers don’t often prescribe steroids for MCAS.

How do I take care of myself?

Ways to take care of yourself with MCAS include:

• Keep a journal of your symptoms. Include information about what your symptoms were and anything you can remember about what you ate or were exposed to at the time. Think about anything that might be different about those times compared to when you haven’t had episodes. Let your provider know if you see any patterns.
• Let close friends and relatives — or anyone who’s around you a lot — know what an attack looks like and where to find your epinephrine injector or other emergency medicine.

When should I go to the ER?

Severe reactions due to MCAS are rare. But if you have symptoms of a serious allergic reaction or anaphylaxis and you have an epinephrine injector, use it. Then, call 911 (or your local emergency service number) or go to the nearest emergency room. Symptoms of anaphylaxis may include:

• Swelling of your face, lips, tongue or throat
• Trouble breathing or swallowing
• Loss of consciousness
• Low blood pressure

What questions should I ask my healthcare provider?

It might be helpful to ask your healthcare provider:

• What should I do if I have an episode/flare-up?
• How do I take my medication?
• How and when do I use my epinephrine injector?
• When should I go to the ER?
• When should I follow up with you?

Does it ever go away?

No, there’s no cure for MCAS. If you have MCAS, you’ll need to manage it with medications.

What’s the life expectancy of someone with mast cell activation syndrome?

Providers expect that most people with MCAS live as long as someone without it. But it’s a newly recognized condition, so we can’t know for sure how it impacts life expectancy.

Is mast cell activation an autoimmune disorder?

No, MCAS isn’t an autoimmune disorder.