What's mast cell activation syndrome?
Mast cell activation syndrome is a condition in which mast cells release their chemical mediators inappropriately, causing recurring symptoms across several body systems. Under the widely used consensus criteria it is diagnosed only when three things are met together: typical recurrent symptoms of mast cell mediator release, an objective rise in blood tryptase during an episode (at least 20% above the person's own baseline plus 2 ng/mL), and improvement on medicines that block or stabilise mast cells.
| Also indexed as | MONDO:0100004 |
|---|---|
| Features mapped | 7 |
| Treatments mapped | 8 |
| Published sources | 8 |
| Last reviewed | 2026-08-04 |
Signs and symptoms
Urticaria
Hives (urticaria) are a common skin sign during episodes of mast cell mediator release.
Angioedema
Deeper swelling of the skin (angioedema), often around the face or lips, can occur during episodes.
Flushing
Sudden flushing, a warm reddening of the skin, is a characteristic feature of mast cell mediator release.
Pruritus
Itching (pruritus) is a frequent skin symptom during episodes.
Gastrointestinal symptoms
The gut is commonly involved, with symptoms such as cramping, nausea, and diarrhoea during episodes.
Recurrent anaphylaxis
Some people have recurrent anaphylaxis, severe whole-body allergic-type reactions, often without an identifiable allergy.
Transient rise in serum tryptase
A temporary rise in blood tryptase during a symptom flare, at least 20% above the person's baseline plus 2 ng/mL, is the objective marker used to confirm that mast cells are activating.
How it is diagnosed
Mast cell activation syndrome
Diagnosed using: Hypotension.
“For instance, mast cell activation syndrome is known as an augmented number of cells due to decreased cell death, resulting in clinical symptoms affecting many systems. The main common symptoms include flushing, hypotension, urticaria, angioedema, headache, vomiting and diarrhea.”
Mast cell activation syndrome
Diagnosed using: Headache.
“For instance, mast cell activation syndrome is known as an augmented number of cells due to decreased cell death, resulting in clinical symptoms affecting many systems. The main common symptoms include flushing, hypotension, urticaria, angioedema, headache, vomiting and diarrhea.”
Mast cell activation syndrome
Diagnosed using: Serum tryptase 20% plus 2 criterion.
“MCAD encompasses MCA syndrome (MCAS) which is defined by (i) documented systemic symptoms of MCA, (ii) a 20% increase in serum tryptase level from the individual's baseline plus 2 ng/mL and (iii) a symptom response to MC-stabilizing drugs.”
Mast cell activation syndrome
Diagnosed using: Episodic multisystem symptoms.
“Symptoms should be severe, episodic, typical of mast cell activation, and involve at least two organ systems.”
Mast cell activation syndrome
Diagnosed using: Consensus diagnostic criteria.
“MCAS is characterized by the presence of typical clinical symptoms, a substantial elevation in serum tryptase levels during an attack compared to the patient's baseline tryptase levels, and a response to MC mediator-targeting therapy.”
Mast cell activation syndrome
Diagnosed using: 24-hour urinary mast cell mediators.
“We recommend measurement of 24-hour urine 11β-PGF₂α and serum tryptase levels of patients with symptoms suggestive of MCAS.”
Treatment and management
What the research describes, not a recommendation. Treatment decisions belong with your clinician.
This covers treatments that appear in the published research mapped here. Investigational and experimental therapies are not included, so their absence is a boundary of this map, not a sign they do not exist.
H1/H2 antihistamines
H1 and H2 antihistamines, which block the effects of histamine released by mast cells, are a first-line treatment and often reduce symptoms substantially; a response to mast-cell-targeting medicines is itself part of the diagnostic criteria.
Used to help with: Mast cell activation syndrome.
“The administration of H1 and/or H2 antihistamines drugs +/- a leukotriene receptor antagonist resulted in a marked reduction in symptoms.”
Mast cell stabilizers
Mast cell stabilizers are one class of medication used when mast cell activation syndrome is suspected, alongside histamine receptor antagonists.
Used to help with: Mast cell activation syndrome.
“When MCAS is suspected, patients can benefit from treatment with histamine receptor antagonists and/or mast cell stabilizers, in addition to avoiding triggers such as certain foods, alcohol, strong smells, temperature changes, mechanical stimuli (eg, friction), emotional…”
Trigger avoidance
Avoiding triggers such as certain foods, alcohol, strong smells, temperature changes, mechanical stimuli, and emotional distress is part of managing mast cell activation syndrome.
Used to help with: Mast cell activation syndrome.
“When MCAS is suspected, patients can benefit from treatment with histamine receptor antagonists and/or mast cell stabilizers, in addition to avoiding triggers such as certain foods, alcohol, strong smells, temperature changes, mechanical stimuli (eg, friction), emotional…”
Omalizumab
Omalizumab can reduce mast cell mediator-related symptoms in people with mast cell activation syndrome that does not respond to standard therapy.
Used to help with: Mast cell activation syndrome.
“The majority of patients with refractory MCAS reported in the literature had a reduction in mast cell mediator-related symptoms with the addition of omalizumab.”
epinephrine auto-injector
An epinephrine auto-injector is used to treat acute episodes of mast cell activation syndrome, particularly when the reaction meets anaphylaxis criteria.
Used to help with: Mast cell activation syndrome.
“Management includes instructing patients to treat acute episodes with an epinephrine auto-injector, particularly when anaphylaxis criteria are met.”
leukotriene receptor antagonist
A leukotriene receptor antagonist can be added to H1 and/or H2 antihistamines for mast cell activation syndrome; in a pediatric-onset cohort, this combination was associated with a marked reduction in symptoms.
Used to help with: Mast cell activation syndrome.
“The administration of H1 and/or H2 antihistamines drugs +/- a leukotriene receptor antagonist resulted in a marked reduction in symptoms.”
cromolyn sodium
Cromolyn sodium, an oral mast cell stabilizer, has been reported to help control signs and symptoms of mast cell activation in people with mast cell activation syndrome.
Used to help with: Mast cell activation syndrome.
“…a cromolyn dose of 1600-2400 mg daily, administered using the continuous oral dosing regimen during the day, was effective in controlling signs and symptoms of mast cell activation.”
aspirin
Aspirin therapy may be used in mast cell activation syndrome for people who have raised levels of the urinary mast cell mediator 11β-PGF₂α, a marker linked with flushing.
Used to help with: Mast cell activation syndrome.
“We recommend aspirin therapy for patients with MCAS and with elevated 24-hour urine 11β-PGF₂α levels.”
How to read the evidence labels
Where this comes from
This guide is built from 8 published source(s). Every claim above links back to one of them. Click any source ID to read the original on PubMed.
Take it further
Printed, source-linked documents built from this condition's graph — ready to bring to an appointment or attach to a coverage request. Every claim carries its published source, the same as this guide.