A Guide for Patients and Families
What is MCAS?
Have you ever felt like you’re “allergic to everything”? Do you experience random hives, stomach problems, headaches, and flushing that seem to come out of nowhere? You might have a condition called Mast Cell Activation Syndrome, or MCAS.
MCAS is a condition where special immune cells in your body — called mast cells — become overly sensitive and release their contents too easily. When this happens, you can experience symptoms throughout your entire body that may seem unrelated but actually share a common cause.
What Are Mast Cells?
Mast cells are part of your immune system. Think of them as tiny security guards stationed throughout your body — in your skin, gut, lungs, and around blood vessels. Their job is to protect you from infections and allergens.
Inside each mast cell are small packets (called granules) filled with powerful chemicals like histamine, prostaglandins, and leukotrienes. When mast cells detect a threat, they release these chemicals to fight it off. This is called “degranulation.”
In people with MCAS, mast cells become trigger-happy. They release their chemicals in response to things that shouldn’t be threats — like certain foods, temperature changes, stress, or even for no apparent reason at all.
What Are the Symptoms of MCAS?
MCAS can affect almost any part of your body because mast cells are found everywhere. Common symptoms include:
Skin:
- Hives (urticaria)
- Flushing
- Itching
- Swelling (angioedema)
Digestive System:
- Nausea
- Abdominal cramping
- Diarrhea
- Food intolerances that seem to change
Heart and Blood Vessels:
- Rapid heartbeat
- Low blood pressure
- Dizziness or lightheadedness
- Fainting or near-fainting
Respiratory:
- Wheezing
- Shortness of breath
- Nasal congestion
Neurological:
- Headaches and migraines
- Brain fog
- Difficulty concentrating
General:
- Fatigue
- Feeling unwell without a clear cause
The key feature of MCAS is that symptoms come in episodes and affect multiple body systems at the same time. You might have a “flare” where you suddenly develop hives, stomach pain, and feel faint all at once.
What Triggers MCAS Symptoms?
People with MCAS often notice that certain things trigger their symptoms. Common triggers include:
- Heat or cold
- Stress or strong emotions
- Certain foods (triggers vary from person to person)
- Alcohol
- Exercise
- Hormonal changes
- Infections
- Certain medications (especially NSAIDs like ibuprofen)
- Physical pressure or friction on the skin
One frustrating aspect of MCAS is that triggers can change over time. A food you tolerated last month might cause a reaction today.
How is MCAS Diagnosed?
Diagnosing MCAS requires meeting three criteria:
1. Symptoms affecting two or more organ systems during episodes
For example, you might have skin symptoms (hives) plus digestive symptoms (nausea) occurring together.
2. Laboratory evidence of mast cell activation
This is measured through blood and urine tests:
- Serum tryptase: A blood test that measures a chemical released by mast cells. Ideally, you need both a “baseline” level (when you’re feeling well) and an “event” level (during a flare). The event level should be significantly higher than baseline.
- 24-hour urine tests: These measure breakdown products of mast cell chemicals, including N-methylhistamine (from histamine), prostaglandin metabolites, and leukotriene E4.
3. Improvement with mast cell-targeted treatments
If your symptoms get better with medications that block mast cell chemicals or stabilize mast cells, this supports the diagnosis.
How is MCAS Treated?
Treatment focuses on blocking the effects of mast cell chemicals and preventing mast cells from releasing them in the first place.
H1 Antihistamines
These block histamine at one type of receptor. Examples include cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra). Many people with MCAS need higher doses than typical allergy sufferers.
H2 Antihistamines
These block histamine at a different receptor, particularly in the stomach and blood vessels. Famotidine (Pepcid) is commonly used.
Mast Cell Stabilizers
Cromolyn sodium helps prevent mast cells from releasing their chemicals. It’s especially helpful for digestive symptoms.
Leukotriene Blockers
Montelukast (Singulair) blocks another chemical released by mast cells.
Avoiding Triggers
Keeping a symptom diary can help identify your personal triggers so you can avoid them.
Emergency Preparedness
People with MCAS who have had severe reactions should carry an epinephrine auto-injector (EpiPen) at all times.
Living with MCAS
MCAS can be challenging to live with, but many people find significant relief with proper treatment. Here are some tips:
- Be patient with treatment. It can take time to find the right combination of medications.
- Keep a symptom diary. Track what you eat, your activities, and your symptoms to identify patterns.
- Communicate with your healthcare team. MCAS is complex and may require care from multiple specialists.
- Connect with others. Support groups can provide valuable information and emotional support.
- Carry medical identification. In case of emergency, responders should know about your condition.
When to Seek Help
If you experience symptoms of a severe allergic reaction — difficulty breathing, throat swelling, severe dizziness, or feeling like you might pass out — use your epinephrine auto-injector if you have one and call 911 immediately.
The Bottom Line
MCAS is a real medical condition that can cause widespread, seemingly unrelated symptoms. If you’ve been told you’re “allergic to everything” or that your symptoms are “all in your head,” know that there may be an explanation — and effective treatments are available.
If you think you might have MCAS, talk to your healthcare provider about getting evaluated. An allergist or immunologist with experience in mast cell disorders can help guide your diagnosis and treatment.
References:
- Weiler CR, Austen KF, Akin C, et al. AAAAI Mast Cell Disorders Committee Work Group Report: Mast Cell Activation Syndrome (MCAS) Diagnosis and Management. J Allergy Clin Immunol. 2019;144(4):883-896.
- Castells M, Butterfield J. Mast Cell Activation Syndrome and Mastocytosis: Initial Treatment Options and Long-Term Management. J Allergy Clin Immunol Pract. 2019;7(4):1097-1106.
- Frieri M, Patel R, Celestin J. Mast Cell Activation Syndrome: A Review. Curr Allergy Asthma Rep. 2013;13(1):27-32.
- Valent P, Hartmann K, Bonadonna P, et al. Global Classification of Mast Cell Activation Disorders: An ICD-10-CM-Adjusted Proposal. J Allergy Clin Immunol Pract. 2022;10(8):1941-1950.
- Theoharides TC, Valent P, Akin C. Mast Cells, Mastocytosis, and Related Disorders. N Engl J Med. 2015;373(2):163-72.
