MCAS and Dysautonomia: How Mast Cells and POTS Connect

What Is MCAS?

Mast cells are immune cells that sit in the skin, the gut and the brain, often right beside nerves. When they're triggered, they release histamine, tryptase, prostaglandins and other chemicals that act on nerves, blood vessels and inflammation (Hasan, 2026). Mast cell activation syndrome (MCAS) means they release those chemicals too easily, in episodes that hit several parts of the body at once. In people with POTS, those episodes have included flushing, shortness of breath, headache, lightheadedness, passing large amounts of urine, and diarrhea, nausea or vomiting (Shibao, 2005).

The consensus criteria ask for 3 things: typical symptoms, a clear rise in tryptase or another mast cell chemical during an episode, and symptoms that improve with medication that blocks those chemicals (Valent, 2012). Specialists still argue about how strictly to apply them. An allergy society work group wrote its criteria to avoid overdiagnosing MCAS in people who likely have other conditions (Weiler, 2019), while the authors of a broader set of criteria argue it's underdiagnosed (Afrin, 2026).

Our team's concern is the loose version of the diagnosis. MCAS has become a diagnosis people are told they "sound like" they have, without a tryptase test or other lab work. If that's where you are, start with an allergist and the labs, because the answer changes what gets treated.

How Are MCAS and POTS Connected?

A 2005 study described people with POTS who had flushing episodes and high levels of methylhistamine, a histamine breakdown product, in their urine. Their episodes were set off by standing for a long time, exercise, meals, the days before a period and sex. On standing, their heart rate climbed from 79 to 114 beats per minute on average, and their systolic blood pressure rose too, which didn't happen in a comparison group with POTS and flushing but normal methylhistamine. That's a hyperadrenergic pattern (Shibao, 2005).

How often the 2 overlap depends on who gets tested. In a 2021 study of 69 people with POTS, 44 had symptoms beyond the usual standing ones, such as migraine, allergies, rashes or gut problems. Of those 44, 29 had at least 1 lab result suggesting mast cell activation, most often raised prostaglandins or histamine markers. That's 42% of the whole group, though the authors note a single abnormal result isn't definitive (Kohno, 2021).

Not everyone accepts the link: a 2020 review found no shared mechanism had been shown between POTS, MCAS and hypermobile Ehlers-Danlos syndrome, and argued that the overlap rests on vague, subjective symptoms (Kucharik, 2020). A 2026 paper in an allergy journal also reported no evidence that MCAS is more common in POTS or hypermobile EDS (Valdez, 2026).

Our team doesn't need to settle that debate to help someone. Many people with POTS do have mast cell symptoms, and in our experience the trigger often sits upstream, in how the nervous system is running the response.

How Does Histamine Affect POTS?

Histamine widens blood vessels, and when you heat up, histamine acting on H1 receptors is part of what opens the vessels in your skin (Wong, 2004). When skin vessels open during a flare, more of your blood sits near the surface. Standing pulls blood toward your legs at the same time, so less reaches your brain, and the heart speeds up to protect brain blood flow. In our view, the racing heart during a flare is that compensation at work.

Do Mast Cells and the Nervous System Drive Each Other?

They do, because mast cells sit beside nerves in the skin, the gut and the brain, and the 2 signal each other in both directions through neuropeptides, neurotransmitters and cytokines (Hasan, 2026). Stress is one route: physical or psychological stress switches on corticotropin-releasing hormone in the hypothalamus, which can trigger mast cells around the heart's small blood vessels to release their contents (Papadopoulou, 2026).

You've seen the everyday version: someone gets nervous before a speech and their skin flushes red. Our team sees the same thing on a larger scale when the brain's brakes on the sympathetic system aren't working, as in hyperadrenergic POTS. Our team has seen someone become allergic to what felt like the whole world after a car accident, having never had allergies before, and people with head injuries often describe skin redness, a scratchy throat and digestive trouble that started with the injury.

Blood flow is part of it too. When standing leaves parts of the brain short of blood, the areas that normally hold the sympathetic system back lose their grip, and the response grows bigger than the trigger. That's why our team looks at how each brain area is working alongside what its blood flow is doing before deciding what comes first.

MCAS or Dysautonomia: How Can You Tell?

Most people with both have some of each. The pattern of a flare points to where it starts.

What you notice	Where it points
Flushing, hives or throat symptoms after specific foods or other clear triggers, with a lab rise during episodes	The mast cells themselves, so an allergist's workup comes first
Symptoms start when you stand and ease when you lie down, with a heart rate rise of 30 or more beats per minute	Brain blood flow and autonomic control, checked with a tilt test and transcranial Doppler
Skin, throat and gut flare together, and the reactions began after a concussion, an illness or a major stress	The nervous system's control of the response, in our team's experience
Gut symptoms and a racing heart that started with an infection such as SIBO or H. pylori	The infection first, then a recheck of the autonomic side once it's treated

A gut infection makes the immune system work hard, and the heart rate rises with it, much like it does with a fever. The arrow can also point the other way: when autonomic problems change how the gut moves, bacteria can overgrow. If the infection is treated and the racing heart stays, our team looks at the autonomic side next.

Is Histamine Intolerance the Same as MCAS?

They're different problems. Histamine intolerance means your body has trouble breaking down histamine you eat, and researchers say it still needs better ways to define and diagnose it (Comas-Basté, 2020). In MCAS, your own mast cells release too much. In our view, both can add to the same blood flow problem in POTS, since both raise histamine.

The main treatment for histamine intolerance is a low-histamine diet (Comas-Basté, 2020). If you keep cutting foods without a real resolution, our team would look at the brain's control of the response, because treating only the downstream reaction can leave you chasing it.

How Are MCAS and POTS Treated When Both Are Present?

Medications that act on mast cells, including histamine receptor blockers and mast cell stabilizers (Hasan, 2026), can improve quality of life a great deal when MCAS is properly diagnosed (Afrin, 2026), and a response to them is part of the diagnosis (Valent, 2012). One caution matters for POTS: in people with mast cell activation and a hyperadrenergic pattern, beta-blockers should be used with great caution, if at all (Shibao, 2005).

These medications block mast cell chemicals or make the cells less likely to release them. They don't change why the cells are primed to fire so easily, or how well your brain protects its own blood flow when you stand.

That's where our team starts, by measuring brain blood flow on a tilt table with transcranial Doppler and looking at how well the brain is braking the sympathetic system. Then we pick a direction: sometimes we steady blood flow to the brain first so it can tolerate training, and sometimes we train the areas that apply the brakes first so blood returns to the head and the heart rate settles.

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