About MCAS

What is Mast Cell Activation Syndrome?
Mast cells, a type of blood cell, play an important role in the body’s immune system. They reside in all body tissues and form part of the body’s initial defence system. Mast cells react to foreign bodies and injury by releasing a variety of potent chemical mediators, such as histamine, when activated. In a healthy person these chemicals will act beneficially to protect and heal the body, but in a person with MCAS these same chemicals are inappropriately triggered and released and have a negative effect on the body. Amongst the triggers are a variety of different foods, exercise, chemicals, fragrances and stress. Many sufferers struggle to identify their triggers and continue to discover new triggers for many years after diagnosis.

MCAS forms part of a spectrum of mast cell disorders involving proliferation and/or excessive sensitivity of mast cells, it has been identified since 2007. It features inappropriate mast cell activation with little or no increase in the number of mast cells, unlike in Mastocytosis*. MCAS causes a wide range of unpleasant, sometimes debilitating, symptoms in any of the different systems of the body, frequently affecting several systems at the same time. The onset of MCAS is often sudden, affecting both children and adults, sometimes in family groups, mimicking many other conditions and presenting a wide-range of different symptoms that can be baffling for both the patient and their physician. Often there are no obvious clinical signs since MCAS confounds the anatomy-based structure underpinning the traditional diagnostic approach. Very often Mast Cell Activation Syndrome is hiding in plain sight.

National Institute of health video on MCAS


Mastocytosis involves inappropriate mast cell activation AS WELL AS an increased number of mast cells. It is a rare but relatively well known mast cell disorder and is currently easier to diagnose than MCAS.


MCAS presents through a wide range of symptoms in multiple body systems, ranging from digestive discomfort to chronic pain, mental health issues and anaphylaxis.

Some key aspects of MCAS are:

  • The symptoms impact more than one body system.
  • People often experience a dramatic step change in symptoms after, perhaps, years of mild symptoms.
  • The symptoms are often episodic or cyclic and wax and wane with varying degrees of intensity, sometimes worsening over time.
  • There is often histamine involvement and will therefore include typical allergy symptoms such as itching, rashes, swelling, inflammation and vomiting.
  • The triggers are many and varied some easily recognized some not, amongst them environmental chemicals, foodstuffs, heat, cold and exercise.
  • MCAS can present simultaneously in patients who have Ehlers Danlos Syndrome (EDS) a connective tissue disorder, and/or Postural Orthostatic Tachycardia Syndrome (POTS).
Symptoms Table (Table 1 in paper)
Diagnosis of MCAS

There is at present no cure and MCAS is difficult to diagnose with great confidence. Patients often spend decades going in circles visiting many different specialists and achieving no proper diagnosis. There are now some doctors who understand the issues and there are tests that can support a diagnosis. Diagnostic criteria were not agreed until as recently as 2011* by an international group of doctors. The criteria rely on a number of factors: typical clinical symptoms, evidence of mediator release and response to medication, as well as a thorough discounting of any other potential diseases.

The first challenge of suspected MCAS sufferers is to find a doctor who has the time and experience to assimilate a complicated medical history. Note that there is no one definitive test for MCAS. Tryptase is reported by many doctors in the published literature to be an unreliable marker for MCAS.  Recognising this, tests for other mediators such as histamine and prostaglandin have recently become available on the NHS. Other tests can also be used to support a diagnosis. Note that these tests are not definitive, capturing and measuring these chemicals is extremely challenging and if positive other potential causes have to be discounted.  A doctor that will listen carefully to the wide range of symptoms and ask themsleves what could be the unifying factor, is key. At present there are few NHS doctors that have developed the clinical experience to feel confident in making a diagnosis and consequently MCAS patients are often invisible to the health system. In the USA and Germany, clinical tests for a range of mediators have been available for years and there is a great deal of clinical experience and indeed published literature on the subject (see our Resources section). Though these tests are not in themselves complete proof of mast cell involvement, when taken with the criteria, they provide reasonable confidence in a diagnosis. 

* Int Arch Allergy Immunol. 2012;157(3):215-25. doi: 10.1159/000328760. Epub 2011 Oct 27. Definitions, criteria and global classification of mast cell disorders with special reference to mast cell activation syndromes: a consensus proposal.

MCAS Centre USA Doctor’s Presentation

The available treatments for MCAS stabilise the mast cells and mitigate the effects of the chemicals they release, e.g. anti-histamines and mast cell stabilisers. Avoiding triggers is also a key part in coping with this illness. See Support and Resources for publications.

There is a wide range in the variety of patients’ response to treatment. It can often take some time to work out what the best medication and dosage are. An added complication is that many patients suffer adverse reactions to the drugs themselves or to the fillers, colouring and preservatives. With a trial and error approach many patients are successful in moderating their symptoms although quality of life can still be affected.

Recent paper on treatment for MCAS

Become a friend

Sign up to become a Friend of Mast Cell Action so we can keep you up to date on our progress and on how to get involved in our latest campaigns and initiatives. Please click on Data Protection at the bottom of the page to see our Data Protection Information.

Become a friend


Mast Cell Action relies entirely on the generosity of people like you. Please make a donation now and together we can make a difference for sufferers of MCAS.

Make a donation