The Interesting Fact About MCAS, Eczema and the Histamine Blame Game

This is the fully referenced, clinical version of this topic. If you would prefer a more accessible and personal exploration of the same subject, without the research detail, I cover it in those terms over on my Substack — Escape the Eczema Trap. You can read that piece here.

A woman walks into a medical clinic with an 18-month history of generalised pruritus and eczema patches spreading across her chest, back, and limbs. She has tried every antihistamine available. Nothing has worked. Her serum tryptase, the standard test for mast cell involvement, comes back normal. If the tests are to be believed, there is no mast cell problem.

But another doctor runs a different test. Urinary 11β-prostaglandin F2α, a metabolite released by mast cells through a pathway entirely separate from histamine. It comes back at more than three times the upper limit of normal.

She starts ibuprofen. The itch resolves. She stops it. The itch returns. She starts again. It resolves again.

This case, documented by Kesterson et al. in 2018, is not a cure story or a medical miracle.⁶ It is a demonstration of something that doctors and patients dealing with antihistamine-resistant eczema need to understand: mast cells do not only release histamine. And the chemical they release instead may not be represented on the test your doctor ordered.

I am writing this article because MCAS and histamine are the top two most searched for terms on my website. The clients I see with treatment-resistant eczema are struggling to manage their skin and in some cases they have not been able to get the right explanation for what is driving it. Mast cell activation syndrome should be considered and so should the nutritional and lifestyle work that can genuinely make a difference. Not simply as a complement to antihistamines, but as an investigation into what is actually going on.

A system designed to protect, misfiring relentlessly

Mast cells are sentinel cells. They position themselves at the borders between the body and the outside world, particularly in the skin, the gut lining, and the airways, ready to respond to anything that looks like a threat. When they detect one, they degranulate: releasing a rapid burst of chemical signals designed to raise the alarm, recruit other immune cells, and contain the damage.

This is exactly what they are supposed to do. In most people, the response happens exactly as it is supposed to. It is proportionate, targeted, and, most importantly, temporary.

In mast cell activation syndrome (MCAS), the system develops a hair trigger. Mast cells start firing in response to stimuli that should not provoke any reaction at all: a change in temperature, the smell of a particular chemical, a glass of wine, physical pressure on the skin, stress, exercise. The mediator release that follows is physiologically significant. It produces symptoms across multiple organ systems simultaneously — skin, gut, cardiovascular, neurological — often in a pattern that makes it difficult to diagnose.

To be formally diagnosed with MCAS, a patient needs three things: recurrent multi-system symptoms consistent with mast cell mediator release; laboratory evidence of elevated mediators; and clinical response to medications that target those mediators.¹ It is that third criterion, response to treatment, that often becomes the most useful diagnostic tool.

The time between a patient flagging the symptoms and actual diagnosis is one of the most troubling aspects of MCAS. Large patient cohort studies have found a median delay between symptom onset and formal diagnosis of approximately 30 years.² Symptoms frequently begin in childhood and the diagnosis may not arrive until the patient is in their fifties. During those three decades, the skin symptoms, the gut symptoms, the flushing, the reactions that do not fit a neat allergy box, are treated as separate, unrelated problems. Nobody looks for the common thread.

In my practice, I see this constantly. Clients who have spent years accumulating diagnoses that each explain part of their picture without ever explaining all of it.

The skin is almost always involved

Skin is one of the organs most consistently and visibly affected by MCAS, for a simple reason: it is packed with mast cells. Of all the tissues in the body, the skin is one of the most densely populated, because it is one of the main surfaces where threats are encountered.

When those mast cells start misfiring, the effects are hard to miss. In prospective studies, close to 90% of MCAS patients develop dermatographism, the ability to write on the skin with light pressure and watch raised red lines appear.¹ In some cohorts, every single patient presented with cutaneous symptoms of some kind.³ Flushing, urticaria, and widespread itching (pruritus) are reported in the majority of cases.²

Most of these skin reactions are acute. Hives that appear and resolve within hours. Flushing episodes triggered by food or heat. Reactive redness that settles overnight. These are classic histamine-mediated reactions, and they respond, to varying degrees, to antihistamines. In a well-characterised cohort of patients with connective tissue disorders and co-existing MCAS, 88% reported meaningful improvement on H1 blockers.⁴

Chronic eczema, the persistent, inflamed, barrier-disrupted skin that defines atopic dermatitis, is a different picture entirely. And it appears to be driven by a different chemical altogether.

When histamine isn’t the problem

Histamine has been taking up a disproportionate part of the conversation. Over my 12 years in practice I, myself, have published multiple articles about this molecule. But there is something that tends to get lost in the conversation about MCAS: mast cells release dozens of different mediators, not just histamine. Prostaglandins, leukotrienes, cytokines, heparin are all released in varying proportions depending on the type of trigger, the genetic background of the individual, and factors that we still don’t know about.

The proportion matters enormously. Because each mediator does different things to the tissue it reaches.

Research by Ravi et al. tracked a cohort of MCAS patients and found that flushing and pruritus correlated specifically with elevated urinary 11β-PGF₂α, a breakdown product of prostaglandin D2.⁵

Not with histamine. With prostaglandins.

When aspirin, which inhibits prostaglandin synthesis, was used therapeutically, it normalised urinary 11β-PGF₂α in eight of nine patients and produced symptomatic improvement in six of the nine.⁵

This is where the Kesterson case outlined in my introduction becomes significant. The patient’s serum tryptase was normal, which would typically suggest no mast cell involvement. But mast cells can release prostaglandins selectively, even in the absence of significant histamine release, and even without triggering the tryptase elevation that most medical practitioners use as their primary diagnostic marker. In this patient, the mast cells were firing constantly, pouring prostaglandins into the skin.⁶

Antihistamines block histamine receptors. If the mediator responsible is prostaglandin D2, an antihistamine has nothing to block. It is absolutely the wrong tool for the problem.

This is where nutrition enters the picture, and not in a minor supporting role. Prostaglandins are synthesised from arachidonic acid, a fatty acid found predominantly in animal fats, and the balance between omega-3 and omega-6 fatty acids in the diet directly influences how much arachidonic acid is available for that process. A diet high in omega-6 and low in omega-3 tilts the body towards more pro-inflammatory prostaglandin production. Shifting that balance, through oily fish, walnuts, flaxseed, is one of the tools I use clinically when the prostaglandin pathway appears to be the primary driver of a client’s skin symptoms. It does not replace medical management, but it is not peripheral either. It acts on the same underlying mechanism.

Two alarm systems, two completely different skin pictures

A useful way to think about MCAS-related skin disease is to imagine two separate alarm systems in the same building.

The first alarm is histamine-driven. When it fires, you get acute reactions: hives, dermatographism, angioedema, flushing that comes and goes. The alarm is loud and obvious. Antihistamines turn it down.

The second alarm is prostaglandin-driven. When it fires, you get a different picture: persistent itching, chronic inflammatory skin changes, a presentation that looks clinically very similar to atopic dermatitis, but does not respond to any of the treatments that atopic dermatitis is supposed to respond to. This alarm runs quietly in the background, and because it does not trigger the expected blood test results, it is routinely missed.

Both alarms have triggers. Physical exertion, heat, cold, alcohol, stress, chemical exposures, and certain foods have all been documented in the literature.^2,7 What triggers the alarm differs from person to person, which is one reason why a one-size-fits-all approach to management reliably fails.

Food is one of the most important and actionable trigger categories for the clients I work with. Foods that either contain histamine directly such as fermented foods, aged cheeses, alcohol, leftovers stored overnight, or that trigger histamine release, strawberries, tomatoes, certain food additives, are common culprits. But “common” does not mean universal. One of the most important things I can do with a client is help them identify their own specific pattern rather than applying a blanket low-histamine list that may have little to do with their actual triggers.

A cluster analysis of 250 MCAS patients found that dermatological symptom patterns differed significantly according to trigger profiles.⁷ Patients who responded to thermal triggers, particularly heat and cold, showed more diverse skin manifestations including flushing, itch, and acne-like lesions. Those who did not react to thermal triggers showed a lower skin symptom burden overall. This tells us something important: the specific trigger profile shapes the clinical picture. Understanding a client’s individual triggers is not just symptom management, it is a window into what is driving the underlying activation.

In practice, many patients have both alarms going off at once. The picture is confusing and the medical treatment is rarely straightforward.

Who is more susceptible, and why

Some people are more susceptible to MCAS-related skin involvement than others, and a few patterns keep emerging.

Connective tissue disorders appear frequently alongside MCAS, with around 41% of patients in survey studies reporting them as comorbidities.⁴ Patients with hypermobile Ehlers-Danlos syndrome or hypermobility spectrum disorders are particularly over-represented: in one prospective study of this population, 45% reported eczema, substantially above what you would expect in the general population.⁸ The structural properties of connective tissue may make cutaneous mast cells easier to activate mechanically, or may amplify the effects of mediator release in skin that already lacks its usual resilience.

Atopic predisposition adds a further layer of complexity. People with atopic dermatitis show an elevated risk of mast cell-mediated angioedema,⁹ and atopy itself, characterised by the IgE overproduction that underlies allergic sensitisation, appears to create a more permissive environment for mast cell activation. The overlap between IgE-mediated atopic inflammation and non-IgE-mediated mast cell activation has not been neatly unpicked in the literature, which I’ve discussed before, and this reflects how genuinely complicated the immune picture is in these individuals.

The gut is part of this picture, and it is consistently underestimated. The gut wall is one of the most mast-cell-dense tissues in the body, which is why so many MCAS patients have significant gastrointestinal symptoms alongside their skin complaints. The relationship runs in both directions: gut dysbiosis and increased intestinal permeability can drive systemic mast cell reactivity, and mast cell activation in the gut lining in turn perpetuates the gut dysfunction. This bidirectional relationship is one of the reasons why gut health is central to the nutritional work I do with clients who have MCAS-related skin conditions. Addressing dysbiosis and intestinal barrier integrity reduces the systemic mast cell trigger load over time, and can shift the overall picture in a way that medical management alone can’t.

Stress is not just a lifestyle factor in this context, it is a physiological one.

Corticotropin-releasing hormone, released during the stress response, directly triggers mast cell degranulation in skin tissue. In clinical practice this means that a client’s flares don’t come out of the blue, even if that is what they appear to do. They often mirror stress events, sleep disruption, and periods of nervous system dysregulation, even when no obvious food or environmental trigger is identifiable. Supporting the stress response is part of the clinical work, not an optional add-on.

The test that misses most cases

This is the part that causes a great deal of clinical frustration. Serum tryptase, the test most commonly ordered when mast cell involvement is suspected, is elevated in fewer than one in four patients with physician-diagnosed MCAS.¹⁰ I think it is worth repeating that. The majority of MCAS patients will have a normal tryptase result, be told there is no mast cell problem, and leave without answers.

However, normal tryptase does not mean the mast cells are behaving. It means the pattern of activation is not tryptase-predominant.

Urinary mediator metabolites tell a more complete story. In the Ravi et al. cohort, urinary 11β-PGF₂α was the most consistently elevated marker, and the one most specifically linked to skin symptoms.⁵ In a separate cluster analysis, 75% of patients showed elevated N-methylhistamine in urine.7 Survey data from 1,574 patients found that 26% reported elevated urinary histamine metabolites and 21.8% reported elevated prostaglandins, but 21.5% of patients with a formal MCAS diagnosis reported no mediator elevation at all.¹⁰ The condition is genuinely heterogeneous, and no single test captures it reliably.

For anyone with antihistamine-resistant eczema where MCAS is a possibility, it is worth asking a GP or specialist specifically for urinary prostaglandin metabolites, particularly 11β-PGF₂α, measured during or shortly after an active symptomatic period rather than at a routine baseline appointment. It is a reasonable request, and the result is likely to be more informative than tryptase alone.

Before testing even becomes the conversation, many people have already accumulated meaningful clinical data about their own pattern through careful observation. That documented pattern is genuinely useful, both for medical investigation and for the nutritional work that runs alongside it.

What evidence do we really have?

As ever it would be wrong to overstate what the research has directly established.

The evidence specifically linking mast cell degranulation to chronic eczematous dermatitis in MCAS is, at this point, almost non-existent. It rests primarily on one detailed case report⁶ and prevalence data from hypermobility cohorts.⁸ Most MCAS research has concentrated on acute urticarial symptoms, which are more dramatic, more easily measured, and more obviously mast-cell mediated. Whether chronic eczema-like changes represent a distinct MCAS skin phenotype driven by prostaglandin-predominant activation, or whether they reflect pre-existing atopic dermatitis being aggravated by mast cell activity in susceptible individuals, has not been definitively answered. The study designs available simply do not let us fully answer that question yet.

What the evidence does support is this: skin symptoms in MCAS are nearly universal; some of those skin symptoms resist antihistamines because they are prostaglandin-driven rather than histamine-driven; and in those cases, targeting prostaglandin synthesis, through prescribed aspirin, NSAIDs, or upstream mast cell stabilisation, can produce improvement where antihistamines have failed.⁶ Nobody has formally studied this in a large, prospective, controlled trial, but the mechanistic logic holds well.

That is where the research stands. And as is so often the case, more needs to be done.

How I approach this with clients

When someone comes to me with chronic eczema that has not responded to antihistamines, particularly where it sits alongside other symptoms such as gut, cardiovascular or neurological, MCAS is always in the differential I am considering.

The first thing I want to understand is the root trigger. What sets off the alarm, how often, in what combinations, and under what conditions. Not just food but also environment, stress, sleep quality, physical activity, chemical exposures, hormonal patterns. Keeping a detailed, structured diary across several weeks is the foundation of everything that follows, because the patterns it uncovers shapes every nutritional and lifestyle recommendation.

From there, the nutritional work typically covers several overlapping areas.

Dietary trigger identification and reduction comes first. Rather than applying a generic low-histamine template, I work with a client to identify their specific pattern. Some people react strongly to fermented foods. Others to alcohol. Others to histamine-releasing foods like strawberries or tomatoes. Some people have very few food triggers and the primary drivers are environmental or stress-related. Getting this wrong, by eliminating everything just in case, creates nutritional insufficiency and food anxiety, both of which worsen the underlying picture. Getting it right requires time and systematic work.

Dietary fat balance is relevant where the prostaglandin pathway appears to be driving the skin picture. Shifting the omega-3 to omega-6 ratio through oily fish, walnuts, and flaxseed is one of the most evidence-grounded nutritional levers available for reducing pro-inflammatory prostaglandin production, and it is something I incorporate as appropriate.

Gut health is almost always part of the work. The gut-mast cell connection means that supporting microbiome diversity, reducing intestinal permeability where that is a factor, and identifying any gut-specific triggers is not peripheral to the skin picture. It is often central to it. This typically involves increasing dietary fibre variety, addressing dysbiosis through food and sometimes targeted supplementation, and pacing the work so changes are tolerable for a reactive gut.

Specific nutrients matter too. Quercetin, found in onions, apples, capers, and green tea, has natural mast cell stabilising properties and is a tool I use regularly alongside dietary work. Vitamin C supports the enzyme that breaks down histamine in the gut, and has mild antihistamine properties in its own right. Magnesium and B6 are cofactors in histamine metabolism pathways that tend to run low in people living with chronic inflammatory stress. None of these are magic solutions. All of them can contribute meaningfully to a calmer baseline.

Stress and nervous system support is part of every MCAS case I work with. Because stress directly triggers mast cell degranulation through neuroimmune pathways, managing the physiological stress response is clinical work, not lifestyle advice. What that looks like varies: for some clients it means structured rest and activity pacing to avoid post-exertional flares, for others it means addressing chronic sleep disruption, for others it means building consistent practices that support nervous system regulation. Identifying what matters most for each person is part of the assessment.

Alongside all of this, medical management has an important role. Mast cell stabilisers, including ketotifen and sodium cromoglicate, reduce mediator release more broadly and showed clinical benefit in 71% of patients in one cohort.⁴ Omalizumab produced significant and sustained improvement in symptom control over a year in patients with idiopathic MCAS.³ Aspirin and NSAIDs are relevant for prostaglandin-predominant presentations. These are decisions made with a GP or immunologist, and I work alongside medical management rather than in place of it.

If this sounds like your picture

If you are an adult with chronic eczema that has not responded to antihistamines, and particularly if that skin picture sits alongside symptoms in other organ systems, you are not out of options.

It is worth observing, before any appointment, whether your skin symptoms tend to flare alongside gut symptoms, flushing, or cardiovascular reactions. Whether aspirin or ibuprofen has ever improved the itch in a way antihistamines did not and that is worth mentioning explicitly to your GP or specialist. Whether hypermobility, a connective tissue diagnosis, or a family history of unexplained multisystem reactions is part of your background.

On the medical testing side, urinary prostaglandin metabolites measured during a symptomatic period are more likely to be informative than tryptase alone, and it is reasonable to ask for them specifically.

On the nutritional side, this is the work I do. If you are considering a more thorough investigation into what is driving your skin, I am happy to have an informal conversation about whether working together would be useful. You can book that here.

If you would prefer to work through this at your own pace, I have put together a self-guided online programme — the Histamine & MCAS Protocol — which takes you through the process systematically, from mapping your individual trigger picture through to the specific dietary, nutritional, and lifestyle changes most likely to make a difference. The programme is available for £125 and you can find out more here.

This is the fully referenced, clinical version of this topic. If you would prefer a more accessible and personal exploration of the same subject, without the research detail, I cover it in those terms over on my Substack — Escape the Eczema Trap. You can read that piece here.

References

The clinical claims in this article are drawn from the following peer-reviewed papers. Hyperlinks to be added at publication so each source can be checked directly.

1. Hamilton M, Hornick J, Akin C, et al. Mast cell activation syndrome: a newly recognized disorder with systemic clinical manifestations. Journal of Allergy and Clinical Immunology. 2011.
2. Afrin L, Self S, Menk J, Lazarchick J. Characterization of Mast Cell Activation Syndrome. American Journal of the Medical Sciences. 2016.
3. Igde MH, Korkmaz P, Toprak I, et al. Idiopathic mast cell activation syndrome in real-life practice: clinical features and management. Allergy and Asthma Proceedings. 2025.
4. Srivastava P, Walsh S, Vadas P. Clinical features and response to treatment in patients with skin biopsies showing congenital connective tissue disorders. Journal of Allergy and Clinical Immunology. 2020.
5. Ravi A, Butterfield J, Weiler C. Mast cell activation syndrome: improved identification by combined determinations of serum tryptase and 24-hour urine 11β-prostaglandin2α. Journal of Allergy and Clinical Immunology: In Practice. 2014.
6. Kesterson K, Nahmias ZP, Brestoff JR, et al. Generalized pruritus relieved by NSAIDs in the setting of mast cell activation syndrome. Journal of Allergy and Clinical Immunology: In Practice. 2018.
7. Häder T, Molderings G, Klawonn F, et al. Cluster-analytic identification of clinically meaningful subtypes in MCAS: the relevance of heat and cold. Digestive Diseases and Sciences. 2023.
8. Mushib C, Moss C, Krapf J, et al. MCAS symptoms among patients with genitopelvic pain and hypermobility. Journal of Sexual Medicine. 2025.
9. Zając M, Bożek A, Grzanka A. The occurrence of atopy in patients with isolated spontaneous mast cell (or nonallergic) angioedema. Journal of Clinical Medicine. 2024.
10. Jennings SV, Slee VM, Hempstead J, et al. The Mastocytosis Society Mast Cell Activation Syndrome patient perceptions survey. Journal of Allergy and Clinical Immunology. 2019.