What is Mast Cell Activation Syndrome?

Mast cells are white blood cells that are part of your immune system and are found throughout your body. They release up to 200 pro-inflammatory mediators (histamines, cytokines, prostaglandins, leukotrienes, etc.) that help your body fight pathogens and allergens.

When you have a mast cell disorder, you may have an abnormally high number of mast cells (Mastocytosis) or you may have a normal number of mast cells that become hypersensitive and hyperactive (MCAS). With MCAS, your mast cells constantly release mediators when they shouldn’t and essentially don’t “turn off,” causing you to be in a chronic allergic state. Mast cells can be triggered by anything and everything – food and beverages, chemicals, stress, temperature changes, exercise, air pollutants, medications, altitude, supplements, and hormone fluctuations (women often experience a flare in symptoms around their menstrual cycle).

Symptoms vary depending on the person and can affect any system in the body. They can include things like anaphylaxis, flushing, hives, gastro-intestinal issues (abdominal pain, diarrhea, and bloating), low blood-pressure, brain fog, headaches, and heart arrhythmias.

MCAS is typically diagnosed by a mast cell specialist or allergist/immunologist using different tests, including  (1) bone marrow biopsies; (2) serum tryptase levels; and (3) urine levels of histamine, prostaglandins, and leukotrienes.  But testing is tricky, as your lab results can come back normal if you are not in a mast cell flare at the time the test is performed. Because of this, some people are diagnosed with MCAS based on symptomology, especially if their symptoms improve when taking antihistamines.


No. Histamine intolerance (HI) and MCAS are two different conditions that share similar symptoms and sometimes overlap. Often, but not always, a person with MCAS also has HI.  Unlike MCAS, which causes your mast cells to become overactive and release mediators (beyond just histamine), HI is when you have trouble breaking down histamine (particularly when you eat) and it accumulates in your body and causes symptoms.

Histamine is mainly broken down by two enzymes: Diamine Oxidase (DAO) in the small intestine and Histamine N-methyltransferase (HNMT) in the central nervous system. HI typically occurs when your body does not produce enough DAO, which limits your ability to break down dietary histamine, especially if you eat too many high-histamine foods. 


The traditional medical protocol for MCAS generally uses a combination of high-dose antihistamines, mast cell stabilizers, and leukotriene inhibitors. The most common medications used are:

  • Histamine 1 (H1) blockers: Claritin, Benadryl, Zyrtec, Allegra, Atarax
  • Histamine 2 (H2) blockers: Pepcid, Tagamet, Zantac
  • Mast cell stabilizers: Cromolyn Sodium, Gastrocrom, Nasalcrom. Ketotifen (also an H1 blocker)
  • Leukotriene inhibitors: Singulair, Zyflo, and Accolate
  • Other medications: Xolair (neutralizes antibodies), low-dose naltrexone (reduces T-cell dysfunction), and aspirin (a prostaglandin inhibitor that may cause reactions in some people, especially those with salicylate sensitivities)