An Overview of Mast Cell Activation Syndrome

Mast cell activation syndrome (MCAS) is a disorder in which mast cells, a normal type of cell found in your immune system, release excessive amounts of histamines and other chemicals that lead to allergy-like symptoms in your body.

People living with MCAS may experience common allergic symptoms like itching and sneezing. They also may have thyroid changes, heart rate and blood pressure fluctuations, fatigue, and nausea. The underlying cause isn't always clear and MCAS can be difficult to diagnose.

This article explains what is known about MCAS and its symptoms and causes. It presents how healthcare providers continue working toward a consensus on MCAS diagnosis and treatment.

Mast cell
Verywell / Nusha Ashjaee

Symptoms of Mast Cell Activation Syndrome

The symptoms of mast cell activation syndrome can affect various systems of the body. These systems can include:

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:

However, their presence does not necessarily indicate a MCAS disorder. Additional research is needed to understand the connection these illnesses may have with mast cells.

What Causes MCAS?

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 (no clearly identified cause).

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
  • Systemic, which can impact many systems of the body

MCAS and Genetics

Research suggests there may be a genetic predisposition to the illness. For example, studies have identified a condition called hereditary alpha tryptasemia (HAT) that some researchers now consider to be a genetic biomarker (diagnostic indicator) in certain people with MCAS.It's also more common in people with systemic mastocytosis than in healthy populations.

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 these causes is extensive, but it can include:

  • Venoms from bites/stings belonging to 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.


Diagnosis of MCAS can be difficult. Currently, there isn’t a test that decisively indicates whether or not someone has MCAS. However, key diagnostic criteria have been identified although new views continue to emerge.

A healthcare provider who is familiar with MCAS may be able to diagnose an individual based on the 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—all of which occur together and may either resolve spontaneously or with treatment.

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. Lab tests can help support the diagnosis, with tryptase levels in the blood generally considered the most reliable.

Other factors that help a healthcare provider to 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

Mast Cell Activation Syndrome Treatment

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 hydrocortisone ointment/cream or Tylenol (acetaminophen). Avoid nonsteroidal anti-inflammatory drugs like Advil (ibuprofen), which can make skin symptoms worse.
  • If symptoms progress to a moderate level of intensity, H1 antihistamine blockers such as Claritin (loratidine), Zyrtec (cetirizine), or Allegra (fexofenadine) may be tried.
  • Other antihistamines—known as H2 blockers—like Pepcid (famotidine) 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 Intal (cromolyn} 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. You may need to carry an EpiPen in case of anaphylaxis.

Note that there may be other kinds of medications not mentioned here that your healthcare provider 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.

Online groups and forums are available for sharing treatment strategies, resources, and support. Joining a group can help you to learn what has helped others and what may be helpful to you, too.


With mast cell activation syndrome (MCAS), the mast cells in your body function abnormally. This leads to excessive allergic-like response with many potential causes, including genetic changes identified by the evolving research.

MCAS can lead to a wide range of symptoms, including body-wide reactions, that can be difficult to diagnose. The development of specific diagnostic criteria continues to progress.

MCAS treatment is focused on stabilizing the mast cell behavior through medication, including the use of H1 (histamine) blockers and corticosteroids.

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. If you’ve been diagnosed with MCAS or suspect you have it, The Mast Cell Disease Society offers a healthcare provider database. It can help you locate a specialist.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Jenny Lelwica Buttaccio, OTR/L
Jenny Lelwica Buttaccio, OTR/L, is a licensed occupational therapist and advocate for patients with Lyme disease.