Mast cell activation syndrome (MCAS)

Mast cells are part of your body's defence system. They sit in the tissues that meet the outside world (your skin, gut, and airways) and act like tiny alarm stations. When they sense a real threat, such as an insect sting or an allergen, they release chemicals like histamine and tryptase. These chemicals cause the redness, swelling, itching, and other reactions that help the body respond.

In MCAS, mast cells release these chemicals when they shouldn't, or release too much. Because mast cells are found all over the body, the chemicals can affect several systems at the same time, for example your skin, stomach, and blood pressure together.

This is why an MCAS episode often looks like an allergic reaction even when there is no clear allergy: flushing, hives, stomach cramps, and lightheadedness can all come on together and then settle again.

MCAS is one of a group of mast cell disorders. It is different from mastocytosis, where the body makes too many mast cells. In MCAS the number of mast cells is usually normal, but they are "too easily triggered”.

MCAS tends to come in episodes, symptoms flare, then ease, and often involve two or more parts of the body at once. Symptoms vary a lot between people, but common ones include:

• Skin: flushing (sudden redness, often of the face and chest), hives, itching, swelling
• Stomach and gut: cramping, abdominal pain, nausea, vomiting, diarrhoea
• Heart and circulation: lightheadedness, palpitations, drops in blood pressure, faintness
• Airways: wheezing, shortness of breath, throat tightness, nasal congestion
• Whole-body: in severe episodes, a full anaphylactic reaction

Many people also report tiredness, headache, and brain fog, though these are less specific and can have many other causes.

A key point: MCAS is not diagnosed from vague, day-to-day symptoms alone. The pattern that points to MCAS is recurrent, more severe, anaphylaxis-like episodes affecting several organ systems together.

Episodes are often set off by a trigger. Reported triggers include:

• Heat, hot water, or sudden temperature changes
• Alcohol
• Certain medicines and, sometimes, contrast dye used in scans
• Physical stimuli such as pressure, friction, or exercise
• Emotional stress
• Hormonal changes
• Infections
• Certain foods
• Insect stings

Keeping track of what comes before an episode can help you and your doctor spot your personal triggers.

MCAS is grouped by doctors into a few types depending on the cause:

• Primary (clonal) MCAS: the mast cells carry a genetic change. This type overlaps with mastocytosis.
• Secondary MCAS: driven by another condition, most often a true IgE-mediated allergy (for example, severe reactions to foods or insect stings).
• Idiopathic MCAS: no trigger, mutation, or allergy is found.

Some people also carry an inherited trait called hereditary alpha-tryptasemia, which raises baseline tryptase and can make mast cells more reactive.

It is worth knowing that MCAS is genuinely overdiagnosed. Many people are told they have MCAS based on long lists of common symptoms without the objective testing that the condition actually requires. A careful assessment protects you from both missing a real diagnosis and being labelled with the wrong one.

Anyone can develop MCAS, but it is diagnosed more often in women (roughly a 3:1 ratio). It is also seen more often in people who have:

• A history of allergies or anaphylaxis
• Mastocytosis or hereditary alpha-tryptasemia
• Hypermobility or hypermobile Ehlers-Danlos syndrome
• Postural orthostatic tachycardia syndrome (POTS) or other forms of dysautonomia

MCAS, POTS, and hypermobility are often discussed together, and some people have more than one. The exact biological link between them is still being studied, and the overlap may partly reflect the fact that all three share many non-specific symptoms.

Doctors use three criteria, and all three must be met for a diagnosis of MCAS:

1. Typical symptoms: severe, recurrent episodes of mast cell activation affecting at least two organ systems — for example skin plus gut, or gut plus low blood pressure.
2. Objective evidence of mediator release: a rise in serum tryptase during an episode, measured against your own baseline. The accepted rule is a rise of at least 20% above baseline, plus 2 ng/mL, measured within a few hours of an episode. Other mast cell chemicals in blood or urine can sometimes be measured too.
3. Response to treatment: symptoms improve with medicines that block mast cell chemicals or calm mast cells.

About the tryptase test. Timing matters. A blood sample should be taken during or shortly after an episode (usually within about 1–4 hours), and a second baseline sample at least 24 hours after everything has settled. A normal tryptase during an episode does not completely rule out mast cell involvement, but the tryptase rise is the most reliable marker available.

Ruling out other causes. Many conditions can cause similar symptoms: allergies, thyroid problems, gut disorders, and dysautonomia among them. Your doctor may check for these, and may look for signs of mastocytosis or hereditary alpha-tryptasemia when appropriate.

There is no cure for MCAS, and no medicine is specifically FDA-approved for it, but most people improve a lot with a stepwise plan. Treatment aims to avoid triggers and to block or calm mast cell chemicals. It is usually built up in steps.

Avoiding triggers. Identifying and avoiding your personal triggers is the foundation of care.

Everyday medicines (first steps).

• H1 antihistamines (for example, cetirizine, fexofenadine, loratadine). These are the mainstay and can often be increased above the standard dose under medical guidance.
• H2 antihistamines (for example, famotidine), especially helpful for stomach symptoms and as an add-on for skin and circulation symptoms.

Added medicines if needed.

• Mast cell stabilisers
• Leukotriene blockers
• Aspirin
• Short steroid courses for severe, hard-to-control flares.

Which combination suits you depends on your symptoms, your triggers, and any other conditions, so the plan should be built together with a doctor.

Small, consistent steps help many people:

• Keep a symptom-and-trigger diary. It helps you spot patterns and shows your doctor how you are doing.
• Take medicines regularly, not only during flares, if your doctor advises preventive dosing.
• Address overlapping conditions. If you also have POTS, allergies, or gut problems, treating those together usually works better than treating each alone.

MCAS varies a lot from person to person. Many people reach good control and a good quality of life once triggers and treatment are sorted out.

If you have signs of a severe allergic reaction (anaphylaxis):

• Difficulty breathing, wheezing, or throat tightness
• Swelling of the lips, tongue, or throat
• A sudden drop in blood pressure, collapse, or fainting
• Widespread hives with feeling very unwell
• A severe reaction after a sting, food, or medicine

Contact your clinic if:

• Your episodes are new, more frequent, or clearly getting worse
• Your current medicines are no longer controlling symptoms
• You have had a reaction and are not sure what triggered it