Mast Cell Activation Syndrome

Mast cell activation syndrome (MCAS) causes your immune system to send repeated false alarms throughout your body, triggering severe episodes of swelling, breathing problems, skin reactions, and digestive symptoms without a clear reason.

Table of contents

• What is mast cell activation syndrome?
• Symptoms
• Causes and triggers
• How is MCAS diagnosed?
• Treatment and management
• Living with MCAS

What is mast cell activation syndrome?

Mast cell activation syndrome (MCAS) is a condition where you experience repeated episodes of severe symptoms that affect multiple parts of your body at the same time. Unlike typical allergies that happen when you’re exposed to something specific, MCAS episodes often occur without any clear trigger.^[1]

To understand MCAS, it helps to know what mast cells are. These are a type of immune cell found throughout your body, in your tissues, skin, lungs, and digestive system. Their normal job is to protect you from harmful invaders like parasites, viruses, and bacteria. When they detect something dangerous, they release chemical substances called mediators (including histamine) that activate your immune system to fight the threat.^[1][2]

In MCAS, something goes wrong with these mast cells. They become defective and release their mediators because of abnormal internal signals, even when there’s no real threat. Sometimes these cells produce too much of these chemicals or release them spontaneously. This inappropriate release of mediators causes symptoms throughout your body.^[2][4]

MCAS is different from other mast cell disorders. While it shares similar symptoms with a condition called systemic mastocytosis, MCAS doesn’t meet specific laboratory criteria used to diagnose mastocytosis. It’s also different from typical allergies because the episodes happen without a clear allergic trigger.^[2][4]

Healthcare providers diagnose MCAS when you have repeated symptoms affecting at least two body systems, laboratory tests show evidence of mast cell activation during episodes, and medications that block mast cell mediators help relieve your symptoms.^[1][2]

Symptoms

MCAS causes repeated episodes of symptoms that can affect many different parts of your body. For a diagnosis of MCAS, symptoms must involve two or more body systems at the same time. These systems include your skin, breathing system (lungs and airways), heart and blood vessels, and digestive system (stomach and intestines).^[1]

Common symptoms include low blood pressure, flushing (hot, red or pink skin), itchy skin, and hives (raised, itchy bumps on your skin). Many people experience nasal congestion and swelling, particularly of the face, lips, eyes, tongue or throat (called angioedema). Digestive symptoms can include constipation or diarrhea (sometimes alternating between the two), abdominal pain, and vomiting. You may also feel short of breath, weak, or faint.^[1][7]

In severe cases, MCAS can cause anaphylaxis, a life-threatening allergic reaction. This is a medical emergency that can make it hard to breathe and cause your blood pressure to drop to dangerously low levels. If you experience severe anaphylaxis symptoms, call emergency services immediately.^[1]

If you’ve ever had an allergic reaction, you have some idea of what mast cell activation feels like. However, for people with MCAS, episodes are often more severe than typical allergic reactions and involve multiple parts of the body happening together. The symptoms can be variable and often change over time, which can make it difficult to identify specific triggers.^[1][5]

Causes and triggers

The exact cause of MCAS is not clearly understood. In some cases, it appears to run in families, suggesting there may be genetic factors involved. Sometimes the condition is caused by genetic mutations in the mast cells, which can produce populations of identical cells that overproduce and spontaneously release mediators.^[4][5]

Although MCAS episodes may seem random, many people identify certain triggers that set off their symptoms. Common triggers include foods and environmental chemicals, fragrances, exercise, stress, and changes in temperature. Other triggers can include bacteria, parasites, viruses, insect stings, wounds, and certain medications.^[5][15]

Triggers vary greatly between individuals, and the same person may find their triggers change over time. This makes identifying specific triggers challenging. What’s more, the number of triggers and severity of symptoms may increase as the condition progresses.^[5]

How is MCAS diagnosed?

Getting a confirmed diagnosis of MCAS can be complex and may take many years. The diagnosis requires completing several stages to rule out other conditions and confirm that mast cells are the source of your symptoms.^[5]

First, your healthcare provider will determine whether you have repeated episodes of symptoms that affect two or more body systems at the same time. These symptoms should be typical of anaphylaxis or mast cell mediator release.^[2][3]

Second, you’ll need laboratory testing during acute episodes to show a temporary rise in a mast cell mediator. The most commonly measured mediator is tryptase, an enzyme mostly found in mast cells. Your doctor may also test for other mediators like histamine. These tests must be done during or shortly after symptoms occur, which can be challenging.^[2][5]

Third, your symptoms must respond to medications that either target mast cell mediators (like antihistamines) or prevent mast cells from releasing their mediators. Finally, your doctor must rule out other conditions that could explain your symptoms.^[2][3]

It’s important to work with a specialist, such as an allergist (a doctor who specializes in allergic conditions), who can provide a complete evaluation and guide you through the diagnostic process.^[1]

Treatment and management

Treatment for MCAS focuses on controlling symptoms and preventing mast cell activation. In most cases, effective therapy consists of medications that block mast cell mediators or stabilize mast cell membranes, supplemented with treatments targeted at specific symptoms and complications.^[4]

The foundation of treatment typically includes antihistamines. Your doctor may prescribe both H1 antihistamines (which help with itching, hives, and swelling) and H2 antihistamines (which help with digestive symptoms and can work together with H1 blockers). These medications work by blocking histamine, one of the main mediators released by mast cells.^[4][10]

Other medications that may be added include mast cell stabilizers (such as sodium cromoglicate), which help prevent mast cells from releasing their mediators. Leukotriene receptor antagonists block another type of mediator released by mast cells. For people who don’t respond well to these medications, your doctor might consider other options like omalizumab or other targeted therapies.^[10][13]

Because severe episodes can progress to anaphylaxis, many people with MCAS need to carry emergency medication. This typically includes an epinephrine auto-injector (also called adrenaline), which you can use immediately if you develop severe symptoms. Your doctor will teach you when and how to use this medication.^[1]

Living with MCAS

Managing MCAS involves more than just taking medications. An important part of living with this condition is identifying and avoiding your personal triggers whenever possible. Keeping a detailed diary of your symptoms, foods, activities, medications, and environmental factors can help you and your doctor identify patterns.^[5][18]

Many people with MCAS find that dietary changes help manage their symptoms. A low-histamine diet, which limits foods that are high in histamine or trigger histamine release, may reduce symptoms. Foods to potentially avoid include aged or fermented foods, alcohol, cured meats, canned fish, certain fruits (berries and citrus), nuts, chocolate, and foods with preservatives. Working with a dietitian who understands MCAS can help you develop a safe, balanced eating plan.^[15][16]

Beyond diet, reducing your exposure to environmental triggers is important. This might include avoiding temperature extremes, using fragrance-free personal care products, removing sources of mold from your home, and managing stress. Some people find that air purifiers help reduce exposure to airborne triggers in their homes.^[15][18]

Stress reduction is particularly important because stress can activate mast cells and cause them to release mediators. Finding ways to manage stress through relaxation techniques, adequate sleep, and maintaining a regular routine can help reduce the frequency and severity of episodes.^[15]

Living with MCAS can be challenging, and it’s important to share information about your condition with family, friends, and coworkers. Let them know about your triggers and what to do if you have a severe reaction. Having a support network who understands your condition can make daily life easier and safer.^[18]