What are Mast Cells?

What are mast cells?

Mast cells are a type of blood cell which are found in all body tissues. These cells play a key role in the immune system. In the event of an injury or an infection, mast cells become active and release chemicals called mediators. In a healthy state, mast cell mediators create a number of protective responses in the body to help fight infection and promote healing.

The table shows how these mediators can affect the body during this response.

Symptoms of MCAS

The symptoms of MCAS are often unpleasant, sometimes debilitating and can include life-threatening anaphylaxis. These symptoms usually involve multiple systems of the body and can include skin (dermatologic) symptoms, gastrointestinal symptoms (such as abdominal pain, nausea and vomiting), respiratory symptoms, neurological problems and urinary issues.

MCAS symptoms are often episodic or wax and wane (meaning that they go and come back) and their severity can fluctuate over time. In some cases, people may suffer mild symptoms for a long time before experiencing a sudden increase in the frequency or severity of symptoms. Possibly after a particularly triggering incident for the immune system, such as a severe infection or a medical procedure or operation.

The extent of symptoms for people suffering MCAS can be substantial. The potential severity of symptoms paired with the unpredictability of triggers can mean that the everyday lives of people with MCAS, their families and carers are considerably impacted.

*These symptoms particularly affect children with MCAS

Recognising anaphylaxis

Anaphylaxis refers to a serious allergic or hypersensitivity reaction which can affect multiple body systems and can range from mild to severe (potentially life-threatening). The specific triggers of anaphylaxis vary between individuals and in some cases the trigger may be unclear (these cases are referred to as ‘idiopathic’).

Common triggers include foods, stinging insect venoms and drugs. Given the sudden onset and potentially fatal consequences of anaphylaxis, it is important to quickly recognise the typical signs and symptoms of these reactions.

Anaphylaxis is highly likely when the following signs are present:

  1. Rapid onset (within minutes to hours) of symptoms involving the skin or mucosal tissue (skin lining the body cavities e.g. inside the mouth and nose) for example, swollen lips or tongue
  2. Breathing difficulties
  3. Reduced blood pressure, collapsing, incontinence (lack of bladder control)
  4. Persistent gastrointestinal symptoms such as cramps, abdominal pain or vomiting


Observational studies and patient-reported data show ‘clustering’ of MCAS with other conditions, meaning that people who have MCAS often also have at least one additional condition. Common comorbidities include connective tissue disorders, e.g. Ehlers Danlos Syndrome (EDS) and Marfans, dysautonomia, such as postural orthostatic tachycardia syndrome (POTS), and type 2 diabetes. The relationship between MCAS and many of these comorbidities is yet to be fully understood, and more research is required.

Further information on comorbidities can be found in the Royal College of GPs Ehlers Danlos Syndromes Toolkit

Further reading:

Akin C. Mast cell activation syndromes. J Allergy Clin Immunol. 2017;140(2):349-355.

Molderings GJ, Haenisch B, Bogdanow M, Fimmers R, Nöthen MM. Familial occurrence of systemic mast cell activation disease. PLoS One. 2013;8(9):e76241. Published 2013 Sep 30.

Theoharides TC, Tsilioni I, Ren H. Recent advances in our understanding of mast cell activation - or should it be mast cell mediator disorders?. Expert Rev Clin Immunol. 2019;15(6):639-656.

Afrin LB, Ackerley MB, Bluestein LS, et al. Diagnosis of mast cell activation syndrome: a global "consensus-2". Diagnosis (Berl). 2020;/j/dx.ahead-of-print/dx-2020-0005/dx-2020-0005.xml.

Simons FER, Ardusso LR, Bilò MB, et al. International consensus on (ICON) anaphylaxis. World Allergy Organization Journal.

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