What Is MCAS? Mast Cell Activation Syndrome Explained

By the OmneDiem Education Team · Updated October 16, 2026 · 9-minute read

What Mast Cells Do (and What Goes Wrong in MCAS)

Think of mast cells as the body's security guards. They start out in the bone marrow, then travel through the bloodstream and settle into tissues at the borders where you meet the outside world: your skin, the lining of your gut, your airways and lungs, and the tissue around blood vessels and nerves. Cleveland Clinic describes them as the body's alarm system.¹ Each one is packed with tiny storage sacs (granules) holding chemical messengers called mediators.

When a mast cell senses a threat, such as an allergen, an injury, or an infection, it releases those mediators. That is a normal, protective response. Researchers have identified a large number of mast cell mediators, including:

• Histamine, which is linked to flushing, itching, and swelling*
• Tryptase and other protein-cutting enzymes; tryptase also serves as a useful lab marker
• Prostaglandin D2 (opens in a new window) and leukotrienes, which affect blood vessels and airways. Prostaglandin D2 is one of the main allergy-related messengers mast cells make, acting through receptors found on immune cells.¹⁶
• Cytokines (opens in a new window) and chemokines (opens in a new window), which call in and direct other immune cells
• Heparin (opens in a new window), which helps keep blood from clotting

Mast cells do not destroy germs directly. Instead, they sound the alarm and recruit other immune cells. They also help with everyday upkeep, such as tissue repair and blood vessel growth.¹

In MCAS, mast cells appear to release these mediators too easily, too often, or in response to things that would not normally set them off. Because mast cells are found throughout the body, the effects can show up in several places at once.

MCAS vs. Histamine Intolerance: What's the Difference?

MCAS and histamine intolerance can look alike, and both involve histamine, so they are easy to mix up. A simple way to picture the difference is a kitchen sink.

Histamine intolerance is generally described as a mismatch between the histamine you take in, mostly from food and drink, and your body's ability to break it down. A key enzyme in the digestive tract, diamine oxidase (DAO) (opens in a new window), handles much of that breakdown. If DAO activity is low, histamine from meals can build up. Learn more in Histamine Intolerance Is Real (opens in a new window).

Dr. DAVE CANDY: A closer look at histamine intolerance

Watch on YouTube (opens in a new window)

Dr. Dave Candy: "Why Healthy Foods Make You Sick." Views expressed are the creator's own, are shared for general education, and do not represent OmneDiem or replace advice from your healthcare provider. This video focuses on histamine intolerance, not MCAS.

MCAS is different but has some key overlapping aspects. The body's own mast cells release mediators, and histamine is only one of them. That is why MCAS episodes can involve more body systems and may not line up neatly with meals. If you are still sorting out whether your reactions look more like allergies or a histamine issue, our guide Allergy or Histamine? (opens in a new window) walks through the basics.

Common Signs Reported With MCAS

Because mast cells are spread throughout the body, people with MCAS describe a wide range of experiences. The hallmark is that symptoms tend to come in episodes and affect two or more body systems at the same time.*⁶

What Is an MCAS Flare?

An MCAS flare is a period when mast cells release a burst of mediators and symptoms ramp up, often in several body systems at once. A flare may come on within minutes of a trigger or build over a few hours, and it can last anywhere from a short while to several days. Many people describe a flare as a cluster, such as flushing plus a racing heart plus stomach upset, rather than a single symptom.*

Flares matter for diagnosis, too. Clinicians often ask for blood or urine samples during or shortly after a flare, because that is when mediator levels are most likely to show a change. For a closer look at what a flare can feel like and how people track them, read What Is an MCAS Flare? (opens in a new window)

What Can Trigger Mast Cell Activity?

Triggers vary a lot from person to person, and some people never find a clear one. Commonly reported triggers include:⁶

• Temperature extremes or quick shifts, including hot showers, saunas, sudden cold, or abrupt weather changes
• Exercise or friction on the skin, such as snug waistbands or tight clothing
• Emotional or physical stress, including poor sleep or illness
• Alcohol. See Histamine Reactions and Alcoholic Beverages (opens in a new window) and What Alcohol Is Highest in Histamine? (opens in a new window)
• Certain foods and food additives, including foods high in histamine (opens in a new window) and foods known as histamine liberators (opens in a new window)
• Strong scents or smells (opens in a new window), such as perfumes, smoke, or cleaning products
• Insect stings
• Some medications, such as NSAID pain relievers, opioids, and certain imaging contrast dyes (always review medications with your prescriber)^6,8. A European expert position paper on clonal mast cell disorders notes that reactions to these drug groups have been reported, while also stressing that the evidence is limited and that many people tolerate them. Medication decisions should always be individualized by your care team.
• Hormonal shifts, such as those around the menstrual cycle or perimenopause or menopause.

A simple symptom journal, noting what you ate, where you were, and how you felt, can be one of the most useful things to bring to your appointment.

The Hormone and Mast Cell Connection

Many women notice their reactions change with their cycle, during pregnancy, or as they move into perimenopause. Research on why is still developing, but scientists have found that mast cells carry receptors for reproductive hormones, so hormone shifts may influence how readily they respond.^{a (opens in a new window),b (opens in a new window)}

Estrogen may turn up the volume

Laboratory research suggests that higher or rapidly rising estrogen may prompt mast cells to release histamine and other inflammatory messengers. Estrogen has also been associated with lower DAO activity, which could mean histamine lingers longer. Adding to the puzzle, some studies indicate histamine may encourage the ovaries to produce more estrogen, which could create a loop that feeds itself.^{a (opens in a new window),b (opens in a new window),c (opens in a new window)} We explore the estrogen and DAO side of this story in The Histamine Hormone Trend (opens in a new window).

Progesterone may act as the brake

Progesterone, by contrast, appears to have a steadying effect on mast cells in research settings. During perimenopause, progesterone often declines and swings unpredictably, so that calming influence may weaken right when estrogen is fluctuating the most.^{d (opens in a new window),a (opens in a new window),b (opens in a new window)}

When perimenopause and mast cell symptoms overlap

In online perimenopause communities, many women describe flares that seem to blend into, or look just like, their hormonal symptoms.^{e (opens in a new window)} Experiences that can show up on both lists include:

Approaches to discuss with your healthcare provider

If hormones seem to play a role in your symptoms, these are topics clinicians and menopause specialists commonly explore. Each one belongs in a conversation with your provider rather than a do-it-yourself plan.^{a (opens in a new window),g (opens in a new window),i (opens in a new window)}

• Hormone therapy. Some clinicians consider progesterone or carefully monitored menopausal hormone therapy, often introduced slowly, as part of an individualized plan.
• Antihistamine medications. Prescribers may recommend combining H1 and H2 antihistamines to help lower the body's overall histamine response.
• Food choices. Cutting back on high-histamine foods such as aged cheeses, cured meats, and red wine, and caring for gut health, may help keep the total histamine load lower. See Foods High in Histamine (opens in a new window).
• Calming the nervous system. Stress management, slow breathing practices, and consistent sleep may help reduce stress-related flares.

Section sources: a. Mast Cell Action: Women's health and MCAS (opens in a new window); b. Dr. Karen Osborne: Mast cell activation and female hormones (opens in a new window); c. Natural Allergy Treatment: What is MCAS? (opens in a new window); d. Dr. Mary Claire Haver (Substack) (opens in a new window); e. r/Perimenopause community discussion (opens in a new window); f. Health in Menopause: Histamine intolerance (opens in a new window); g. The Pause Life: Mast cells, histamine, and the hormone connection (opens in a new window); h. Clinic 51: Histamine intolerance and menopause (opens in a new window); i. Doctor Anat: Hormones and histamine (opens in a new window).

How Would I Know If I Have MCAS?

You can't confirm MCAS at home, and there is no single test for it. Instead, clinicians look for a specific combination of findings. Under widely used consensus criteria, including guidance from the American Academy of Allergy, Asthma & Immunology (AAAAI), a diagnosis generally requires all three of the following:^2,3

1. Recurring flares that involve two or more body systems at once. The episodes keep coming back, and each one affects at least two of these areas at the same time:⁶
   • Skin: sudden flushing with heat and redness, hives, itching, swelling under the skin (angioedema), or dermatographism, where a light scratch or pressure leaves raised red lines*
   • Heart and circulation: low blood pressure, lightheadedness, fainting, or a fast heartbeat*
   • Digestion: intense stomach cramping, nausea, vomiting, or diarrhea*
   • Breathing: wheezing, coughing, shortness of breath, or a stuffy, itchy nose*
2. Lab tests showing a spike in mast cell chemicals. Blood or urine collected during a flare is compared with a sample taken on a day you feel well. The most common marker is serum tryptase, ideally drawn within about 30 minutes to 4 hours after symptoms begin. A frequently used threshold is a rise of 20% + 2 ng/mL over your baseline. Urine tests for breakdown products such as N-methylhistamine, prostaglandin D2 metabolites, or leukotriene E4 may also be ordered, sometimes through specialized single-sample urine panels.⁷ Mayo Clinic Laboratories, for example, offers a random urine panel that checks all three of these markers. Because the sample can be collected at home after a flare, it can be a practical add-on when a timely tryptase blood draw isn't possible.¹⁵
3. A clear response to mast cell–targeted medication. Symptoms noticeably improve with prescription or clinician-directed medications that block or stabilize mast cell chemicals, such as H1 and H2 antihistamines or mast cell stabilizers.*

Symptoms that usually aren't enough on their own

Many conditions share features with MCAS, including fibromyalgia, chronic fatigue syndrome, anxiety disorders, and isolated food sensitivities. For that reason, clinicians generally do not diagnose MCAS based only on:¹

• Ongoing tiredness without allergy-like or multi-system episodes*
• Widespread body or joint aches by themselves
• Brain fog or difficulty concentrating as a stand-alone concern*
• Anxiety or mood changes as the main or only symptom*

Underlying causes and related findings

Clinicians also look for what may be driving mast cell activity and may describe MCAS as primary (linked to a change in the mast cells themselves, as seen in mastocytosis), secondary (driven by another condition, such as an IgE-mediated allergy), or idiopathic (no clear cause found). Some people also carry a genetic trait called hereditary alpha-tryptasemia (HαT), in which extra copies of a tryptase gene raise baseline tryptase. Research estimates it is present in roughly 4–6% of people in studied populations.^4,15

What to do next

Because MCAS is complex and can resemble many other conditions, the best starting point is an allergist, immunologist, or another specialist experienced with mast cell disorders. Before your visit, consider keeping a symptom journal and photographing skin changes during flares. For in-depth patient resources, visit The Mast Cell Disease Society (opens in a new window) or the Allergy & Asthma Network (opens in a new window).*

Celebrities With MCAS

Several well-known public figures have spoken publicly about living with Mast Cell Activation Syndrome (MCAS), often alongside related conditions like Ehlers-Danlos Syndrome (EDS) and Postural Orthostatic Tachycardia Syndrome (POTS).^{1 (opens in a new window),2 (opens in a new window),3 (opens in a new window)} Their openness has helped bring attention to a condition that is often called "invisible."

Health details above are summarized from each person's own public statements as reported by the linked sources. Every person's experience is unique. None of the individuals named are affiliated with, sponsored by, or endorse OmneDiem or its products.

Everyday Considerations to Discuss With Your Clinician If You Suspect You Have MCAS or If You Have MCAS

Care for MCAS is personal, and plans are built with a healthcare team. Clinicians commonly use medications that block or stabilize mast cell mediators, alongside lifestyle steps. Questions and habits many people find helpful to raise include:

• Tracking triggers in a journal or app so patterns are easier to spot
• Asking about a lower-histamine eating plan, ideally with a registered dietitian, so nutrition stays balanced. Our histamine causes and tips guide (opens in a new window) covers food basics.
• Managing temperature and stress where possible, such as cooler showers and steady sleep routines
• Reviewing all medications and supplements with your prescriber before making changes
• Having an emergency plan if your clinician recommends one

DAO Supplementation and MCAS

Let's be clear about what DAO enzymes (opens in a new window) can and cannot do. DAO is a digestive enzyme that helps break down histamine from food in the gut.* It does not act on mast cells, and it is not a treatment for MCAS. For people who notice reactions to histamine-rich meals, some clinicians discuss DAO as one way to support the body's natural breakdown of dietary histamine.* If you have MCAS, talk with your healthcare provider before adding any supplement.

Explore OmneDiem DAO Supplements (opens in a new window)

Frequently Asked Questions About MCAS

Keep Learning

• Allergy or Histamine? How to Tell the Difference (opens in a new window)
• What Are Histamine Liberators? (opens in a new window)
• What Is an MCAS Flare? (opens in a new window)
• What Is Diamine Oxidase (DAO) Enzyme? (opens in a new window)
• Foods High in Histamine (opens in a new window)
• Histamine Intolerance Is Real (opens in a new window)

Related Resources

• The Mast Cell Disease Society: Symptoms and Triggers of Mast Cell Activation (opens in a new window)*
• Allergy & Asthma Network: Mast Cell Activation Syndrome (opens in a new window)
• Cleveland Clinic: Mast Cells (opens in a new window)
• Cleveland Clinic: Mast Cell Activation Syndrome (opens in a new window)

References

1. Cleveland Clinic. Mast Cells: Anatomy, Function & Diseases. my.clevelandclinic.org (opens in a new window)
2. Valent P, et al. Definitions, criteria and global classification of mast cell disorders with special reference to mast cell activation syndromes: a consensus proposal. Int Arch Allergy Immunol. 2012. PubMed (opens in a new window)
3. Weiler CR, et al. AAAAI Mast Cell Disorders Committee Work Group Report: Mast cell activation syndrome (MCAS) diagnosis and management. J Allergy Clin Immunol. 2019. PubMed (opens in a new window)
4. Lyons JJ, et al. Elevated basal serum tryptase identifies a multisystem disorder associated with increased TPSAB1 copy number. Nat Genet. 2016. PubMed (opens in a new window)
5. Maintz L, Novak N. Histamine and histamine intolerance. Am J Clin Nutr. 2007. PubMed (opens in a new window)
6. The Mast Cell Disease Society. Symptoms and triggers of mast cell activation. tmsforacure.org (opens in a new window)
7. Allergy & Asthma Network. Mast cell activation syndrome (MCAS). allergyasthmanetwork.org (opens in a new window)
8. Bonadonna P, Pagani M, Aberer W, et al. Drug hypersensitivity in clonal mast cell disorders: ENDA/EAACI position paper. Allergy. 2015;70(7):755–763. doi:10.1111/all.12617 (opens in a new window)
9. Cleveland Clinic. Mast Cell Activation Syndrome (MCAS). my.clevelandclinic.org (opens in a new window)
10. The Mast Cell Disease Society. Mast cell activation syndromes. tmsforacure.org (opens in a new window)
11. Review of mast cell activation syndrome diagnosis and laboratory testing. PubMed Central. PMC10672129 (opens in a new window)
12. POTS UK. Mast cell activation syndrome (MCAS). potsuk.org (opens in a new window)
13. Patient Power. Mast cell activation triggers. patientpower.info (opens in a new window)
14. Mast Cell Action. MCAS triggers. mastcellaction.org (opens in a new window)