Mast cell activation syndrome (MCAS) is a condition where mast cells, a type of immune cell, release chemicals such as histamine too easily, or at the wrong time.
Mast cells are found all over the body, including the skin, gut, airways and blood vessels. Normally, they help protect you from infection and cause the symptoms of an allergic reaction. In MCAS, they can react to everyday triggers, causing symptoms that come and go in different parts of the body.
Symptoms
Symptoms often affect 2 or more parts of the body at the same time. They can include:
- skin: flushing, itching, hives (a raised, itchy rash) or swelling
- gut: tummy pain, feeling or being sick, diarrhoea or bloating
- heart and circulation: a fast heartbeat, low blood pressure, dizziness or fainting
- airways: a blocked or runny nose, wheezing or shortness of breath
- whole body: tiredness, headaches and brain fog
Symptoms can be mild or severe, and they can change from day to day.
Common triggers
Triggers are different for everyone. Common ones include:
- heat, or sudden changes in temperature
- alcohol
- some foods
- stress, strong emotions or pain
- exercise or tiredness
- infections
- hormone changes, such as periods
- strong smells, such as perfume or smoke
- some medicines, such as anti-inflammatory painkillers
Linked conditions
MCAS is sometimes seen in people who also have hypermobile Ehlers-Danlos syndrome (hEDS) or postural tachycardia syndrome (PoTS). Some people with long COVID have similar symptoms. Researchers are still studying how these conditions are linked.
How MCAS is diagnosed
There is no single test for MCAS. A specialist, usually an immunologist or allergy specialist, will usually look for 3 things:
- typical symptoms that come in episodes and affect 2 or more parts of the body
- blood or urine tests showing a rise in mast cell chemicals during a flare. A common test measures tryptase in your blood, within about 4 hours of symptoms starting, and compares it with your usual level
- symptoms that get better with medicines that target mast cells, such as antihistamines
Other conditions with similar symptoms need to be ruled out first.
Getting help
See your GP if you have symptoms like these that keep coming back. They can check for other causes, start some treatments and refer you to a specialist if needed.
Read about MCAS treatment and living with MCAS.
Common questions
Is MCAS the same as an allergy?
No. In an allergy, the immune system reacts to one specific thing, such as peanuts or pollen, and allergy tests are usually positive. In MCAS, mast cells can react to many different triggers, and allergy tests are often normal.
Is MCAS the same as mastocytosis?
No. In mastocytosis, the body makes too many mast cells. In MCAS, the number of mast cells is usually normal, but they release their chemicals too easily.
Can MCAS be cured?
There is no cure, but treatment and avoiding your triggers can help control symptoms for many people.
Sources we used
- Mast Cell Action, UK charity for people with MCAS
- Valent and others, Diagnostic criteria for mast cell activation syndrome (consensus proposal)
- Leeds Teaching Hospitals NHS Trust, Diagnosis of MCAS
- PoTS UK, Mast cell activation syndrome
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