Overview of Mast Cell Activation Syndrome

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To understand mast cell activation syndrome (MCAS), you must first have a basic understanding of mast cells. Everyone has mast cells—they are immune system cells that are responsible for how your body reacts to allergens. If you come into contact with an irritant, either internal or external, mast cells produce an allergic reaction.

The reason you experience allergy-type symptoms when exposed to a trigger is that mast cells release chemical mediators (such as histamine and tryptase) when you are in contact with foreign substances. They defend against infections, injury, and play a vital role in the body’s inflammatory processes. In a healthy individual, mast cells serve to protect and help the body heal.

However, for reasons that aren't completely understood yet, mast cells can become faulty and release excessive amounts of chemical mediators, often to multiple stimuli, resulting in a barrage of allergic-type symptoms affecting various systems of the body.

Mast cell
Verywell/Nusha Ashjaee


MCAS was first identified in 2007, as stated by Mast Cell Action, a nonprofit organization based in England and Wales that seeks to provide support, education, and resources to patients, physicians, and policy-makers.

Although more research is needed to determine why and how someone may develop an overactive mast cell response, here’s what we do know about the symptoms that a person may experience.

A review in the journal Current Allergy and Asthma Reports suggests that MCAS may impact multiple systems of the body, including:

The symptoms associated with MCAS can range from only a few to a sweeping set of unpleasant issues, but they typically involve two or more organ systems of the body. The symptoms include:

  • Anaphylaxis
  • Itchy skin, rashes, or hives
  • Swelling of the skin or mucous membranes (may be seen in severe allergic reactions)
  • Wheezing
  • Flushing or reddening of the skin
  • Red eyes
  • Gastrointestinal issues (nausea, vomiting, diarrhea, cramping, or food sensitivities)
  • Low blood pressure
  • Fainting
  • Tachycardia or rapid heart rate
  • Nasal congestion

MCAS symptoms may flare up on a cyclical basis, vary in severity, and change over time.

People with this condition may have other overlapping illnesses as well, such as Ehlers-Danlos syndrome (EDS), postural orthostatic hypotension (POTS), interstitial cystitis (IC), irritable bowel syndrome (IBS), and others, but additional research is needed to understand the connection these illnesses may have with mast cells.


At the present time, MCAS is thought to be a family of disorders with several potential causes. These are classified as primary, secondary, or idiopathic. Research indicates there may be a genetic predisposition to the illness. Approximately 74 percent of people with disorders affecting mast cells have one or more first-degree relatives with a mast cell condition as well.

In primary MCAS, the cause is an abnormality in the mast cells themselves. One such disorder is mastocytosis, which causes an abnormal amount of mast cells to accumulate in different parts of the body. The two types of mastocytosis are cutaneous, which primarily affects the skin, and systemic, which can impact many systems of the body.

Monoclonal MCAS is another primary disorder, in which an abnormal clone of mast cells causes the symptoms.

In secondary MCAS, the mast cells themselves are normal, but they become abnormally activated by an external stimulus. People with secondary MCAS have triggers that provoke an exaggerated immune response. The list of triggers is extensive, but it can include one or more of the following:

  • Venoms from bug bites (bugs such as bees, spiders, and flies)
  • Changes in weather temperature
  • Food
  • Alcohol
  • Exercise
  • Medications
  • Chemicals
  • Infections
  • Stress

In idiopathic MCAS, abnormal mast cell activation occurs without any identifiable, consistent trigger, and no primary mast cell disorder can be identified. ("Idiopathic" means "of unknown cause.")


Currently, there isn’t a test that decisively indicates whether or not someone has MCAS. However, three pieces of key diagnostic criteria have been identified in Current Allergy and Asthma Reports.

First, a physician who is familiar with MCAS may be able to diagnose an individual based on her clinical presentation when two or more organ systems of the body are impacted. For example, a person with MCAS may have an increased heart rate, a rash, and experience vomiting—many combinations of symptoms are possible.

The second is that a person who experiences noticeable relief from their symptoms when given medications—like H1 or H2 histamine blockers that help stabilize mast cells—supports the idea that the patient may be exhibiting signs of MCAS.

Finally, during flare-ups of MCAS, a person may have elevated urine or blood levels of chemical mediators like tryptase or histamine during two or more episodes. This lab test can help support the diagnosis of MCAS.

Other factors that help a physician reach a diagnosis of MCAS are:

  • taking a detailed medical history
  • completing a thorough evaluation
  • ruling out other medical conditions that may cause a similar set of signs and symptoms
  • routine monitoring to watch for the development of other diseases


To date, there is no cure for MCAS. The primary goals of treatment are to stabilize mast cells so that they cease to release their chemical mediators, provide relief of symptoms, and minimize known triggers. Everyone reacts to treatment differently, so it may take a bit of trial and error to find what works for you.

If your symptoms are mild (such as a headache or itchy skin) you may be able to ease them with over-the-counter medications like ibuprofen or hydrocortisone ointment or cream. If symptoms progress to a moderate level of intensity, H1 antihistamine blockers such as diphenhydramine (Benadryl), hydroxyzine (Vistaril), or loratadine (Claritin) can be useful to combat itchiness, pain or discomfort in the belly, and flushing or redness on the skin.

Other antihistamines—known as H2 blockers—like ranitidine (Zantac) and famotidine (Pepcid) can reduce gastrointestinal upset and lessen nausea. Both types of antihistamines help to minimize the release of the chemical mediator histamine.

Other mast cell stabilizers such as cromolyn or ketotifen may work to help lessen symptoms as well. In severe cases of MCAS, corticosteroids may be recommended to mitigate the ongoing cascade of chemical mediators and inflammation. In the case of anaphylaxis, you may need to carry an EpiPen to administer a shot of epinephrine.

Note that there may be other kinds of medications not mentioned here that your doctor may prescribe you for your symptoms of MCAS.

Regardless of what medications you take, being mindful of your triggers and doing your best to avoid them is best practice.


MCAS can cause a considerable amount of distress for the people who have it, and the illness can be lonely and isolating. Sometimes, no matter how hard you try to avoid potential triggers, the condition flares up anyway—it may seem like no one understands what you’re going through.

There are plenty of online groups and forums where patients gather to share treatment strategies, resources, and to support one another. Joining a group can help you feel like you’re not alone and also provide you with valuable information to locate an expert skilled in treating diseases with mast cell involvement. You can learn what has helped others and what may be helpful to you, too.

If you’ve been diagnosed with MCAS or suspect you have it, The Mastocytosis Society provides a Physician Database to help you locate a specialist, and hopefully, get you on the path to feeling better.

A Word From Verywell

Though there is no known cure for MCAS and the condition can be very frustrating to cope with, rest assured that you can connect with other people who are going through similar experiences. Discuss with your doctor the best way to manage your symptoms—you can connect with The Mastocytosis Society to contact a physician who is knowledgable about your condition.

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