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A Deeper Understanding of Mast Cell Activation Syndrome
If you’re living with Mast Cell Activation Syndrome (MCAS), you know the frustration all too well, the unpredictable symptoms, the diagnostic odyssey, and the constant search for answers. Flushing, fatigue, gastrointestinal distress, and a cascade of seemingly unrelated issues can make daily life feel like navigating a storm.
But why are these symptoms rooted in your tiny but mighty mast cells?
In this in-depth interview, Dr. Lawrence Afrin, a leading expert in mast cell disorders and hematologist-oncologist, shares groundbreaking insights into MCAS biology, its typical genetic underpinnings, and why it might overlap with conditions like hypermobile Ehlers-Danlos Syndrome (hEDS) and Postural Orthostatic Tachycardia Syndrome (POTS), among others.
Whether you’re undiagnosed, newly diagnosed, or have been living with MCAS for years, this post will provide you with a deeper understanding of mast cells and their role in human health and disease.
Let’s dive in by first learning about mast cells.
What Are Mast Cells?
Mast cells are among the body’s first responders, ancient immune cells that have been around for 500 million years, predating many other defense mechanisms. In a healthy body, they detect threats (like infections or injuries) and release a “cocktail” of mediators, chemical signals that rally other cells to fight back and promote healing. Think of it as an instant alarm system: quick, targeted, and ultimately helpful.
Mast Cell Locations In The Body
Unlike many immune cells that freely circulate in the bloodstream, mature mast cells are primarily tissue-resident; they don’t patrol the blood unless they’re immature precursors en route to a specific destination, where they mature and take up permanent posts. This strategic positioning allows them to act as frontline guardians in high-risk areas exposed to the outside world, such as the skin, mucosal linings of the gut, lungs, and nasal passages, or even nestled just outside blood vessels, lymphatic vessels, in virtually every organ. This allows the mast cell to be close to the action, surveying for irritants and intruders.
Remarkably, mast cells also directly abut all neurons in both the peripheral and central nervous systems, positioning them to rapidly detect and respond to neural signals, as well as irritants and intruders. This proximity to the action enables them to serve as a security team for your nervous system.
These white blood cells specialize in rapidly detecting threats within seconds, degranulating to unleash a precise “cocktail” of mediators that orchestrate a swift, coordinated response. This can rally nearby cells to contain the danger, amplify inflammation where needed, and kickstart healing.
How Mast Cells Behave Differently in MCAS
Patients with MCAS have a population of mast cells that misfire. As Dr. Afrin explains, “Normal activation is normal… when there’s a threat, the normal mast cell detects it and responds almost instantly with release of… the mediators that are appropriate for stimulating other aspects of the body to respond… helping the body resist and recover from the insult.”
The central problem in MCAS isn’t usually an explosion of mast cell numbers (though there might be a modest increase in some tissues); it’s inappropriate activation. “When the mast cells start producing and releasing the wrong mediators in the wrong amounts, wrong times, wrong durations, wrong places in the body,” then you develop symptoms that aren’t helpful; they’re harmful.
For patients, this means symptoms like hives, brain fog, or heart palpitations aren’t “just allergies” but a product of this dysregulation. “The symptoms you get [from abnormal activation] that’s not helpful for anything,” Dr. Afrin notes. Understanding the shift from “helpful inflammation” to “dysfunctional chaos” is crucial to comprehending this medical condition.
The Genetic Puzzle: Why MCAS Varies Wildly from Person to Person
One of the most eye-opening parts of the interview is Dr. Afrin’s discussion of MCAS’s heterogeneity and why no two patients experience it quite the same way. “Essentially, every patient with this is unique in their patterns of abnormal mediator production and release,” he says.
Here is what the science suggests is the culprit: somatic DNA mutations. These mutations are not inherited but rather are acquired changes in genes that regulate mast cell behavior. And because the mutations are not inherited, they’re not present in all of the mast cells. Instead, they’re only present in a fraction (and likely a fairly small fraction, at that) of the mast cells, and it’s even readily possible for different sets of mutated mast cells to bear partially or completely different sets of these somatic mutations. In general, these mutations enhance mast cell activation, causing not only an inappropriate baseline level of activation but also an inappropriate response to non-threatening stimuli. “It’s a model very similar to cancer… acquired mutations… driving [an] inappropriate response,” Dr. Afrin describes.
Research by pioneers like Dr. Gerhard Molderings shows most MCAS patients harbor multiple such mutations in a tiny subset of their mast cells, often starting early in life and accumulating over time. Not only is the full assortment of mast cell mutations different from patient to patient, but other key variables also contribute to the heterogeneity of symptoms among MCAS patients.
MCAS Heterogeneity Explained
The harmful mast cells are not evenly distributed throughout your body. For instance, some individuals may have a small population of pathological mast cells in the gut. Others may have a similar dysfunctional subset predominantly in the skin or lungs, leading to organ-specific symptom dominance, such as gastrointestinal flares in the former and dermatological or respiratory issues in the latter.
In addition, each mast cell is inappropriately responding to different stimuli. That stimulus could come from something that is consumed or inhaled, while other stimuli might come from within the body, such as hormonal fluctuations, emotional stress, or even mechanical forces, such as vibration or pressure on tissues.
These variables underscore why diagnosis is tricky: MCAS doesn’t fit neat patterns. “This disease… is going to present in a thousand different ways,” Dr. Afrin warns, challenging traditional medicine’s “pattern recognition” approach, at least when it comes to the individual, clinically apparent (“superficial”) symptoms that patients experience.
Yet, at a higher level, there is an overarching pattern: since most of the mast cell’s mediators trigger effects that fall under broad umbrellas of inflammation or allergic responses, while also shaping growth and development across all body tissues, MCAS typically reveals itself from that “100,000-foot” vantage point as a chronic, multisystem condition marked by recurring themes of inflammation, allergy-like reactions, and subtle (often small, benign, and clinically insignificant) irregularities in growth and development that can crop up in virtually any tissue.
The Diversity of Mast Cells: More Than Allergies
Medical training curriculum often reduces mast cell function to two mediators (tryptase and histamine) and one receptor (for IgE-mediated allergies). Medical doctors and scientists in training are taught that mast cells are involved in allergic responses, in mild forms causing your runny, itchy nose, watery eyes, and sneezing, and, far less commonly, at the worst, causing life-threatening anaphylaxis. Therefore, health professionals tend to have an oversimplified view of mast cell biology.
In reality, mast cells are similar to a Swiss Army Knife, producing (or “expressing”) over 1,200 potent mediators and 300+ receptors. “Each mediator is incredibly potent in a wide range of actions… upon a vast range of other cells and tissues and organs in the body, and the range of receptors gives the mast cells vast abilities to recognize and react to various chemical and other stimuli,” Dr. Afrin reveals. This combinatorial explosion, permutations of abnormal signal responsiveness and abnormal signal expression, creates “an almost incalculable number of variants” found in MCAS.
Evolution wired mast cells as versatile survivors in a toxic ancient world. Today, when mutated, they unleash “wacky” effects across organs. Yet, this potency is a double-edged sword: targeted therapies could one day silence specific misfires.
Dr. Afrin explains there is a logical reason why mast cells are the most functionally diverse immune cell: “This is evolution at work. The mast cell is the oldest immune cell [involved in] host defense, dating back more than 500 million years, when eukaryotic multicellular life was first emerging…the lymphocytes, the neutrophils, the macrophages… all of these more specialized defense cells, more effective at more specific aspects of defense, all came along hundreds of millions of years later, and because of their better abilities at specific defenses, they came to be our dominant immune cells. But the mast cell has persisted and likely has retained most or all of its originally evolved abilities to sense and respond to a very wide array of dangers.”
Doesn’t MCAS Run In Families, Suggesting It’s Inherited?
If you’ve noticed patterns of Mast Cell Activation Syndrome (MCAS) symptoms in your family, like multiple relatives dealing with unexplained allergies, digestive woes, or chronic fatigue, you’re not imagining things. “MCAS is a disease of high familial propensity”, explains Dr. Afrin.
This suggests that something about MCAS is inherited. However, what’s usually inherited is not the genetic mutations found in mast cells. “If you look at the mutations in the mast cells in a family with some members suffering MCAS, we usually find different mutations in the mast cells in different affected members of the affected family. And yet it’s a familial thing,” explains Dr. Afrin.
This suggests “there is something getting inherited,” but it’s usually not the genetic mutations themselves. Instead, the inherited trait “seems to be epigenetic mutations.” To explain what this means, we will first define the term epigenetics.
Epigenetics Simplified
Think of the genes as light switches. The light switch itself is the gene, while the fingers that flip the gene ‘on’ and ‘off’ are proteins that sit down on the DNA and ‘read’ the genetic code. But what happens when the cell does not want the gene to be ‘turned on’?
Your cells have evolved ways to make it very easy or very hard for the light switch (gene) to be flipped ‘on’. Specifically, there is a group of enzymes that will ‘cover the light switch’, making it hard to access, like a childproof light switch cover. Simply put, epigenetics is the ability for the cell to ‘cover the light switch’ or completely ‘remove the cover’. The numerous enzymes responsible for adding and subtracting these molecular covers work in exquisite harmony, orchestrating a symphony of gene expression akin to a maestro guiding a philharmonic orchestra through its most intricate passages.
But what happens when you inherit mutations in the epigenetic enzymes?
Inherited Epigenetic Mutations in MCAS?
Now that you understand epigenetics, the question is how might inherited mutations in genes that encode your epigenetic machinery cause the buildup of somatic mutations in mast cells?
The honest answer is we don’t know. Data suggests that “epigenetic mutations are being inherited…[and] those mutations then facilitate the emergence of somatic mutations that are going to differ from one patient to the next.”
How Might DNA In Those With MCAS Be Mutated?
To put this in context, consider this: each time a cell divides, it must copy its genetic code. This means billions of DNA replication events occur throughout your tissues all day, every day, as cells divide and renew. Although DNA replication is remarkably accurate, errors, or somatic mutations, inevitably slip through. To counter this, our bodies have evolved robust surveillance systems: mechanisms that monitor your DNA for mistakes, either repairing correctable mutations on the spot or triggering apoptosis (programmed cell suicide) to eliminate the faulty cell and prevent the error from spreading.
Somatic mutations not only occur from errors in DNA replication, but also from external and internal assaults on your genetic code. From UV radiation and chemical exposures in the environment to reactive oxygen species and viral infections, there are numerous ways somatic mutations can be introduced into your DNA. This underscores the need for your cells, especially rapidly dividing cells, to keep the genes that encode DNA repair machinery turned ‘on’.
Could Inherited Mutations Cause An Accumulation of Somatic Mutations in Hematopoietic Stem Cells?
If the inherited epigenetic mutations dial down the activity of genes responsible for these detection-and-repair systems, some replication errors will slip past the safeguards, building up unchecked inside the cell. When such a persisting mutation arises in a hematopoietic stem cell (the foundational source for all blood cells, including mast cells) and strikes a gene critical to mast cell regulation, the result can be a clonal population of mast cells all carrying that same mutation. Suddenly, those mast cells are primed to overreact, triggering MCAS symptoms. This work, however, is still evolving, as researchers have yet to identify the mutated epigenetic genes predicted to be inherited by those with MCAS.
MCAS and EDS, POTS and More: Is There A Hidden Link?
If you have MCAS alongside hypermobile EDS (hEDS) or POTS, you’re not alone, and Dr. Afrin believes that somatic mutations in mast cells may be the “root” driver for many such overlaps in a subset of patients with these comorbidities. Mast cell mediators don’t just cause inflammation or allergies; they guide tissue growth and development. Dysregulated signals could explain connective tissue fragility in EDS or autonomic instability in POTS.
“Take a population of MCAS patients who all have hypermobile Ehlers-Danlos syndrome (hEDS)… the odds are that some share mutations which are what’s actually driving the hypermobile Ehlers-Danlos syndrome part,” Dr. Afrin hypothesizes. For decades, researchers have hunted for germline (inherited) mutations in hEDS without success, potentially because the culprits are somatic, hidden within mast cells. As stated earlier, collecting sufficient numbers of mast cells in the blood to identify these somatic mutations is challenging. Mast cells virtually never even show up in a routine blood count “differential”, a laboratory analysis that measures the relative percentages of white blood cell types in a blood sample.
The same logic applies to fibromyalgia, IBS, chronic fatigue syndrome, and hundreds more. “There are… hundreds, perhaps even thousands, of other diseases that are associated with MCAS in which MCAS, in different variants, is probably what’s at the root,” he estimates. Preliminary studies even show mild increases in mast cells in fibromyalgia skin biopsies, but certainly not to the numbers or other specific abnormal patterns seen in the frankly malignant (and rare) mast cell disease called mastocytosis.
This interconnectedness validates your multi-system struggles. It’s not “all in your head,” it could be a web of mast cell-driven effects, yet more research is needed.
Why MCAS Was "Hidden" for So Long—and What's Changing
MCAS isn’t new; it’s newly recognized, with its first reports around 2007. Why the delay? Doctors learn in medical school that mast cell pathology can result in an incredibly rare form of disease called mastocytosis. “Most doctors… never see a single case [of mastocytosis],” Dr. Afrin says, fostering bias against mast cell issues being a prevalent problem. This, combined with the disease’s variability in clinical presentation, poses great challenges for the physician in recognizing the condition, which is so prevalent that “essentially all doctors are likely to observe various cases of it every day”. However, clinicians can’t recognize that MCAS underlies the more superficial clinical issues they are recognizing, diagnosing, and treating.
In fact, it wasn’t until Dr. Afrin had seen his first 70 or so MCAS patients that he found the first one with a “reasonable similarity” to any of the others—highlighting the challenge in recognizing the underlying pattern of the disease when the doctor generally has limited time and can only focus on the more superficial and obvious symptoms and problems. Yet diagnostic tools exist that can detect mediator release, such as tryptase and histamine, following a mast cell flare-up.
The good news? “We already have all the technologies… to figure out how any particular mutation is bringing about the observed clinical effects.” Pilot studies could soon link MCAS mutations to hEDS or POTS phenotypes, sparking targeted treatments.
Dr. Afrin envisions a “cottage industry” of pilot research projects, each examining a small cohort of patients with one of the many chronic inflammatory diseases of unknown cause to identify what proportion of each cohort has identifiable MCAS. If a subset of patients suffering from various chronic diseases shows clinical, biochemical, and/or genetic evidence (such as somatic mutations in key mast cell genes) of MCAS, this could challenge researchers and clinicians to reconsider these illnesses through a mast cell lens, potentially sparking publications with fresh ideas about underlying causes and leading to better patient outcomes.
Hope on the Horizon: Empowering MCAS Patients Today
Living with MCAS can feel isolating, but Dr. Afrin’s message is empowering: Your symptoms have a biological basis, and science is catching up. “It is going to take armies of researchers [and] centuries of effort,” he admits, but the path forward is clear—personalized mutation profiling, mediator-targeted therapies, and holistic management.
In the meantime, knowledge is your strongest ally. Track your triggers using unhide®, advocate for comprehensive testing, and connect with communities like the Brain Inflammation Collaborative.
If you found this blog post informative, please share it with your friends, family, or on social media! Thank you for reading.
This post is based on an exclusive interview with Dr. Larry Afrin, conducted in 2025. Always consult your healthcare provider for personalized advice.
Written By
Matthew Menendez, Ph.D.
Director of Science Communication, Brain Inflammation Collaborative
Dr. Menendez has a passion for empowering patients navigating complex chronic diseases. Through innovative educational strategies, he bridges the gap between cutting-edge research and real-world understanding, helping individuals advocate for better care and access to timely treatments.
