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What is Mast Cell Activation Syndrome?

In our last blog post, we finished off by looking at reasons why you can have histamine mediated symptoms even though you don’t have a histamine intolerance or DAO deficiency.

Mast cell activation syndrome (MCAS) can look just like histamine intolerance.

What are mast cells?

Mast cells are immune cells best known for releasing histamine and other chemicals during an allergic reaction. In addition to allergies, mast cells regulate the rest of the immune system, protect from infection, and are involved in repair tissue—their primary job is to keep you safe.

Mast cell activation syndrome (MCAS) is one type of mast cell activation disorder where mast cells are hyper-reactive and release excessive amounts of chemical mediators. Unlike an allergic reaction, there are various triggers that stimulate the release of chemical mediators. These triggers can be hard (or sometimes impossible) to identify.

These reactions are sometimes referred to as pseudo-allergies, or idiopathic allergies (unknown cause), because the antibodies involved in an allergic response aren’t present. MCAS can occur with or without allergies.

Signs & Symptoms of MCAS


  • You look fine, and bloodwork comes back normal, but you don’t feel fine

  • You react or are sensitive to smells, drugs, herbal supplements, bug bites, or other substances

  • You don’t tolerate temperature extremes or weather changes well

  • You’ve had different symptoms at different times in your life that come and go at random (they wax and wane)

  • You get periodic flares in symptoms, and you don’t know why

  • You have idiopathic allergies where doctors don’t know the cause

  • You have symptoms across different body systems

  • You have difficulty sleeping or always feel tired or fatigued

  • You haven’t felt good for a long time, or maybe ever


  • Migraines

  • Headaches

  • Tinnitus

  • Dizziness

  • Nerve pain

  • Nerve tingling

  • Hives

  • Eczema

  • Itchy skin

  • High or low blood pressure

  • Arrhythmia

  • Heart palpitations

  • Chest pain

  • Nasal congestion or drip

  • Asthma

  • Nausea

  • Heartburn of GERD (reflux)

  • Vomiting

  • IBS

  • Stomach pain

  • Muscle pain

  • Joint pain

  • Fatigue

The list of symptoms is the same as those of histamine intolerance.

Do you have any of these signs and symptoms?

Which condition do you have?

We’ll share some insight into the confusion in future blog posts, coming in October.


New to figuring out the histamine or mast cell piece to the puzzle for yourself, and curious to know more? Would it help to know some steps you can take to get started? We're leading a free class we call The Histamine Connection coming up on September 29th this month. It's free to join! Register here, and make sure to bring your questions to this live class.

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