What is MCAS?

“My body is constantly having an allergic reaction.”

This is my quick response when people ask what is mast cell activation syndrome (MCAS). However, this disease is NOT your typical IgE allergies.

My mast cells are overprotective.

Mast cells are our immune system’s first line of defense. Unfortunately, my mast cells can no longer appropriately determine and respond to threats. They overreact both in times of true danger and when they are not supposed to.

Mast cells can wreak havoc on the entire body.

Mast cells release hundreds of chemicals, such as histamine and prostaglandins, like grenades all over our bodies. These chemicals cause a variety of symptoms such as fatigue, insomnia, hives, migraine, IBS, bone pain, muscle weakness, vomiting, tachycardia, dizziness, PMDD, and anxiety. Symptoms can range in severity from discomfort to disability. Mast cell reactions can cause life-threatening complications, including anaphylaxis.

Certain foods, chemicals, and activities can make MCAS worse.

My mast cells overreact to exercise, fragrances, sunlight, many foods ( FODMAPs, or salicylates), hot or cold temperatures, hormone fluctuations, and stress. High histamine foods are especially challenging when my histamine levels are already high. Everyone has different mast cell triggers.

Tests can help diagnose MCAS.

However, no test, including tryptase, can rule out MCAS and false negatives are common. MCAS blood tests include:

  • Serum tryptase
  • Serum chromogranin A
  • Plasma histamine chilled on ice
  • Plasma PGD2 chilled on ice
  • Plasma PGF2a chilled on ice
  • Plasma heparin chilled on ice

PGD2, PGF2a, leukotriene E4, and n-methylhistamine are best measured by a 24-hour chilled urine test, but you must keep the container cold AT ALL TIMES. Random urines may also be helpful, but the specimen should be kept on ice and frozen immediately. Testing availability varies greatly and is ever-changing.

Treatment can help manage symptoms and prevent damage

Each MCAS patient tolerates medication and supplements differently, but the goal is to reduce and block as many mast cell mediators (chemicals released by mast cells) as possible. For example, mast cells release prostaglandin D2, the strongest inducer of sleep in the human body; medications and supplements that inhibit prostaglandin production can significantly reduce brain fog, fatigue, and other related symptoms.

Please talk to your doctor before adding any medications or supplements, and research all of the ingredients, as inactive ingredients can be triggers for MCAS. Here are a few examples:

  • Cetirizine blocks H1 histamine receptors
  • Famotidine blocks H2 histamine receptors
  • Montelukast sodium inhibits leukotrienes
  • Aspirin inhibits prostaglandins
  • Cromolyn sodium stabilizes mast cells
  • Ketotifen stabilizes mast cells
  • Vitamin C inhibits histamine production
  • Vitamin D stabilizes mast cells
  • Quercetin stabilizes mast cells
  • Turmeric inhibits prostaglandins

What causes MCAS?

The exact cause of MCAS is unknown, but it is commonly believed to be caused by a combination of genetics and environmental triggers, which include:

  • Infections (such as COVID-19 or Lyme disease)
  • Toxins (including mold)
  • Chemicals
  • Physical trauma
  • Psychological trauma

MCAS treatment may help reduce or eliminate symptoms dysautonomia, EDS, and other health conditions

Mast cell activation is often underappreciated when it comes to Ehlers-Danlos syndrome (EDS) and dysautonomia (POTS is one type of dysautonomia). MCAS remission eliminated my dysautonomia and EDS symptoms. Oftentimes, doctors and patients haven’t been educated on mast cell mediators, and blame symptoms on EDS or POTS. For example, mast cell mediators can thin blood, increase blood vessel permeability, and cause vasodilation, which affects circulation. Mast cell stabilizers and other treatments can reduce the mediators that cause these symptoms.

Research on MCAS is currently very limited, but many health conditions are associated with mast cell activation. More research is needed to explore how MCAS treatment may help following health conditions:

  • Gastroparesis or small intestinal bacterial overgrowth (SIBO)
  • Multiple chemical sensitivity
  • Endometriosis
  • Interstitial cystitis (IC)
  • Fibromyalgia
  • Anxiety, premenstrual dysphoric disorder, and other psychiatric disorders

How to find a specialist

Given that MCAS is a relatively newly recognized disease, MCAS specialists are rare and vary in their medical training–for example: hematology, immunology, or internal medicine. Patient groups, such as Facebook support groups, can provide helpful recommendations for finding a specialist near you. Finding a specialist who recognizes MCAS patients often have normal tryptase levels can save a lot of time and money. At this time, travel is often necessary to receive care beyond the first lines of therapy.

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More resources

For a longer read, I highly recommend Never Bet Against Occam: Mast Cell Activation Disease and the Modern Epidemics of Chronic Illness and Medical Complexity by Lawrence B. Afrin. Although many MCAS patients have mild symptoms, this book illustrates the complexity of MCAS.


This website is for educational purposes only and not intended to be a substitute for medical advice, diagnosis, or treatment. Always seek the advice of a qualified medical professional with any questions you may have regarding a medical condition.