The Pattern
People with hypermobile EDS often turn out to have mast cell activation too. The two travel together more often than chance would predict.
Many people who carry a diagnosis of hypermobile Ehlers-Danlos Syndrome (hEDS) eventually find they are also reacting to ordinary exposures — foods, fragrances, temperature shifts, stress — in ways that don't fit a typical allergy pattern.
That second pattern often turns out to be Mast Cell Activation Syndrome (MCAS). The overlap is common enough that researchers have started treating it as a recognizable clinical association rather than a coincidence.
Where Mast Cells Actually Live
Mast cells are abundant inside connective tissue — the same tissue that is altered in EDS.
Mast cells are immune cells that sit throughout the body — in skin, gut lining, airways, blood vessel walls, and most densely, in connective tissue.
Connective tissue is exactly what EDS affects. The collagen that holds joints, organs, and vessels together is built differently in EDS — looser in hypermobile types, more fragile in others.
This puts mast cells inside a structural environment that doesn't behave the way it should. One leading theory is that this altered environment may keep mast cells in a chronically activated state, releasing histamine and other mediators when they shouldn't.
The Collagen Factor
Atypical collagen may act as an ongoing low-level signal to nearby mast cells, keeping them quietly activated.
In hEDS, the body's collagen is not necessarily damaged — it's built differently from the start. Some research suggests that atypical collagen can act as a persistent low-grade signal to nearby immune cells, including mast cells.
If the matrix is constantly flagging that something isn't right, the cells embedded in it may stay in a state of low-level alert. Over time, that low-level alert can begin to express itself as the symptoms recognized as MCAS.
The Tryptase Thread
Hereditary alpha tryptasemia is one of the genetic links currently being investigated in this overlap.
Hereditary alpha tryptasemia (HαT) is a genetic variant that leaves people with elevated baseline blood tryptase — a chemical mast cells release. People with HαT show higher rates of both mast cell activation symptoms and connective tissue findings.
It does not explain every case of the overlap, but it is one of the threads that helps explain why the two patterns travel together more often than chance would suggest.
How Often This Overlap Appears
Estimates suggest up to a quarter of hypermobile EDS patients also meet criteria for MCAS.
Research and clinical reports estimate that up to 25% of people with hEDS also have features consistent with MCAS. This number may underrepresent the overlap, since MCAS is itself underdiagnosed.
The point is not to assume one diagnosis means another, but to notice when the two patterns appear together — because a clinician familiar with one is not always looking for the other.
Shared Symptoms Across Systems
The overlap shows up across digestion, the cardiovascular system, the skin, and the nervous system.
People who have both conditions often report symptoms that don't sit cleanly inside any one specialty:
- Gastrointestinal — nausea, bloating, food intolerances, alternating constipation and diarrhea
- Cardiovascular — racing heart on standing, lightheadedness, near-fainting, flushing
- Neurological — migraines, brain fog, fatigue that doesn't improve with rest
- Skin — flushing, hives, easy bruising, dermatographism (writing on the skin leaves a raised mark)
- Sleep and energy — disrupted sleep, post-exertional crashes, sensory sensitivity
Each of these symptoms is non-specific on its own. The pattern emerges when several show up together and respond to the same triggers.
Why the Overlap Gets Missed
Specialists tend to see what they're trained to see — and the two diagnoses live in different specialty silos.
Joint specialists may diagnose hEDS without thinking of mast cells. Allergists may consider mast cell disorders without checking for hypermobility. Primary care often sees only one symptom at a time, in fifteen-minute slots.
The result is that someone may carry one diagnosis for years before the second becomes part of the conversation. Patients themselves often spot the pattern first — usually after a flare that doesn't fit either condition's standard playbook.
What to Pay Attention To
If you have one of the two, it may be worth noticing whether features of the other are also present.
For someone already diagnosed with hEDS, MCAS-pattern signals can include:
- reactions to foods that don't match a true allergy pattern
- symptoms triggered by fragrances, smoke, weather changes, stress, or heat
- flushing, hives, or itching without a clear cause
- GI symptoms that wax and wane with no consistent food trigger
- multiple symptoms across systems firing at once
For someone diagnosed with MCAS, EDS-pattern signals can include:
- joint hypermobility — loose joints, dislocations, joints that “give”
- soft, stretchy, or unusually thin skin
- easy bruising or slow wound healing
- chronic joint or back pain without injury
- family members with similar joint or skin findings
Bottom Line
The overlap is real, increasingly recognized, and worth bringing to a clinician who looks at both.
The connection between hEDS and MCAS is no longer a fringe idea. It is being studied, written about in clinical literature, and increasingly recognized in patient-facing care.
If you carry one diagnosis and the symptom picture has never quite fit, the second condition is worth raising with a clinician who is willing to consider it. Pattern recognition — by you, with notes — often opens the door.
Related reading: The Trifecta: EDS, MCAS, and POTS — and Why the Building You're In Matters, and MCAS — Next Steps.