Understanding mast-cell mediators, breathing symptoms and diagnostic uncertainty
The Impact of Mast Cell Activation Syndrome on Respiratory Symptoms
Mast Cell Activation Syndrome, usually abbreviated to MCAS, describes recurrent systemic episodes in which mast-cell mediators are released inappropriately. Wheeze, throat tightness, nasal congestion and breathlessness may occur, but these symptoms overlap with asthma, rhinosinusitis, inducible laryngeal obstruction, reflux and several other respiratory conditions.
- Mast-cell mediators
- Wheeze
- Breathlessness
- Throat tightness
- Tryptase testing
- Asthma overlap
- Mastocytosis
- Anaphylaxis
MCAS is a systemic episodic disorder
Under consensus criteria, symptoms should occur in recurrent episodes affecting at least two organ systems and be accompanied by objective evidence of mast-cell mediator release.
Respiratory symptoms are not specific
Wheeze, cough and breathlessness can result from asthma, infection, reflux, upper-airway obstruction, dysfunctional breathing or another condition even when MCAS is suspected.
What is Mast Cell Activation Syndrome?
MCAS is a disorder characterised by recurrent systemic episodes caused by inappropriate mast-cell activation and mediator release. It should not be diagnosed from non-specific chronic symptoms alone.
Mast cells are immune cells found in tissues that interact closely with the external environment, including the skin, gastrointestinal tract and respiratory mucosa. They contain and generate substances such as histamine, tryptase, prostaglandins, leukotrienes, cytokines and chemokines.
These mediators have useful biological roles. They contribute to defence against pathogens, regulation of blood vessels, allergic responses and communication with other immune cells.
During mast-cell activation, mediator release may cause flushing, itching, hives, abdominal cramping, diarrhoea, wheeze, throat symptoms, low blood pressure or anaphylaxis. The exact symptom pattern varies between episodes and patients.
Mast-cell activation can occur during ordinary allergy, anaphylaxis and several recognised diseases. MCAS is diagnosed only when the complete clinical, biochemical and treatment-response criteria are satisfied.
How can mast-cell mediators affect breathing?
Histamine
Histamine can increase vascular permeability, cause tissue swelling, stimulate nasal secretions and contribute to bronchoconstriction in susceptible airways.
Leukotrienes
Cysteinyl leukotrienes can promote bronchial smooth-muscle contraction, mucus secretion and airway inflammation.
Prostaglandins
Prostaglandin D2 and related mediators may contribute to flushing, nasal symptoms, bronchial effects and systemic reactions.
Tryptase
Tryptase is released during significant mast-cell activation and is the best-validated laboratory marker for systemic episodes.
Cytokines and chemokines
These signalling molecules recruit and activate other immune cells and may amplify inflammation.
Mucosal swelling
Increased vascular permeability can contribute to nasal blockage, tissue oedema and, during severe reactions, upper-airway compromise.
Respiratory symptoms associated with mast-cell activation
Respiratory symptoms may appear as part of a broader systemic episode, especially when skin, gastrointestinal or cardiovascular symptoms occur at the same time.
Wheeze
Bronchoconstriction can produce a musical expiratory sound, particularly when asthma or airway hyperresponsiveness coexists.
Breathlessness
Breathlessness may result from bronchospasm, upper-airway symptoms, hypotension, anxiety, hyperventilation or another respiratory condition.
Chest tightness
Tightness can occur with bronchoconstriction but should not automatically be attributed to allergy or MCAS.
Cough
Cough can accompany bronchospasm, postnasal drainage, reflux or airway irritation. Persistent isolated cough is not a specific MCAS feature.
Throat tightness
This may reflect laryngeal oedema during anaphylaxis, inducible laryngeal obstruction, reflux or a sensation associated with dysfunctional breathing.
Nasal congestion
Histamine and other mediators can cause nasal blockage, sneezing, itching and watery discharge.
Postnasal drainage
Rhinitis and sinus inflammation may create a sensation of mucus draining into the throat and can aggravate cough.
Noisy inspiration
Inspiratory noise or stridor suggests upper-airway narrowing and requires urgent assessment when severe or accompanied by swelling.
Persistent cough can be assessed through the London Chest Specialist chronic-cough diagnosis service .
Unexplained breathlessness may require a structured breathlessness assessment rather than attribution to mast-cell activation alone.
MCAS, asthma and breathing-pattern dysfunction
Asthma, mast-cell activation and dysfunctional breathing can coexist, but they are separate diagnoses and require different evidence.
Asthma
Typical pattern
Variable wheeze, cough, breathlessness and chest tightness that change over time or with triggers.
Objective evidence
Spirometry, bronchodilator reversibility, FeNO, peak-flow variability or bronchial challenge may support diagnosis.
Treatment
Corticosteroid-containing inhalers and other asthma-directed therapies are used according to severity.
Inducible laryngeal obstruction
Typical pattern
Throat tightness, inspiratory noise and difficulty breathing in, often with rapid onset and recovery.
Assessment
Laryngoscopy during symptoms or provocation may demonstrate inappropriate laryngeal closure.
Treatment
Breathing and laryngeal-control therapy are usually more important than escalating asthma medication.
Breathing-pattern dysfunction
Typical pattern
Air hunger, frequent sighing, chest or upper-body breathing and symptoms out of proportion to objective lung findings.
Triggers
Illness, pain, stress, asthma and fear of allergic reactions may reinforce an inefficient pattern.
Treatment
Respiratory physiotherapy, pacing and treatment of underlying respiratory disease may help.
Read about objective asthma diagnosis and treatment .
The role of airway inflammation testing is explained in the FeNO testing guide .
Further information is available in the guide to breathing-pattern dysfunction .
Response to an inhaler does not diagnose MCAS
Bronchodilators can improve bronchospasm from asthma or another obstructive airway disorder. MCAS requires its own systemic and biochemical diagnostic evidence.
Other causes of similar respiratory symptoms
| Condition | Possible symptoms | How it may be distinguished |
|---|---|---|
| Asthma | Wheeze, cough, chest tightness and breathlessness | Objective evidence of variable airflow obstruction or type-2 airway inflammation |
| Inducible laryngeal obstruction | Throat tightness, inspiratory noise and sudden dyspnoea | Laryngeal examination during symptoms or provocation |
| Allergic rhinitis | Sneezing, itching, congestion and postnasal drainage | Allergen history, examination and selected allergy testing |
| Chronic rhinosinusitis | Nasal blockage, facial pressure, mucus and reduced smell | ENT examination, nasal endoscopy or sinus imaging |
| Gastro-oesophageal reflux | Cough, throat clearing, hoarseness and chest discomfort | Clinical assessment and targeted reflux investigation |
| Respiratory infection | Cough, sputum, fever, wheeze and breathlessness | Clinical course, microbiology and chest imaging |
| Pulmonary embolism | Sudden breathlessness, chest pain, tachycardia or collapse | Urgent probability assessment, blood tests and imaging |
| Cardiac disease | Breathlessness, chest pressure, palpitations and fainting | ECG, cardiac biomarkers, echocardiography or other cardiac tests |
| Panic or hyperventilation | Air hunger, tingling, dizziness and chest tightness | Diagnosis only after important respiratory and cardiac causes have been assessed |
Does MCAS increase the risk of respiratory infections?
Mast cells participate in innate immune defence, but current evidence does not establish MCAS itself as a general immune deficiency that routinely causes recurrent respiratory infection.
Mast cells can recognise microbial signals and release mediators that recruit other immune cells, increase vascular permeability and promote mucus production. These responses may help contain pathogens but can also contribute to inflammatory symptoms.
During a viral or bacterial respiratory infection, inflammation may aggravate asthma, rhinitis, urticaria or other mast-cell-associated symptoms. This does not necessarily mean that MCAS caused the infection.
Asthma exacerbation
Viral infection can worsen bronchial inflammation and trigger wheeze independently of systemic MCAS.
Increased mucus
Infection and inflammatory mediators can increase nasal and bronchial secretions.
Medication effects
Corticosteroids or other immunosuppressive medicines used for a separate condition may increase infection susceptibility.
Alternative immune disorders
Frequent bacterial infections may require investigation for antibody deficiency, bronchiectasis or impaired mucus clearance.
Recurrent or unusually severe infection can be assessed through the specialist chest-infection service .
Daily sputum production or recurrent infection may also justify assessment for bronchiectasis .
How is Mast Cell Activation Syndrome diagnosed?
Widely used consensus criteria require all three components: characteristic recurrent systemic episodes, objective evidence of mast-cell mediator release and improvement with appropriate mast-cell-directed treatment.
- Document recurrent systemic episodes Symptoms should occur together in at least two organ systems, such as skin plus respiratory symptoms or gastrointestinal symptoms plus low blood pressure.
- Record the timing and severity Clinicians assess whether episodes resemble anaphylaxis and whether they require urgent treatment.
- Measure acute serum tryptase A sample is ideally obtained as soon as practicable during a significant episode, commonly within one to four hours of onset.
- Measure baseline tryptase A separate baseline sample is taken after complete recovery, usually at least 24 hours after symptoms have settled.
- Apply the event-related tryptase formula A significant rise is commonly defined as acute tryptase greater than baseline multiplied by 1.2, plus 2 ng/mL.
- Consider selected urinary mediators Specialist testing may include urinary N-methylhistamine, leukotriene E4 or prostaglandin metabolites, although collection and interpretation are less standardised.
- Assess response to treatment Symptoms should improve with treatment that blocks mast-cell mediators or stabilises mast cells.
- Exclude alternative diagnoses Allergy, asthma, mastocytosis, hereditary alpha-tryptasaemia, endocrine disease, cardiac disease and other mimics are assessed.
| Test | Possible role | Important limitation |
|---|---|---|
| Acute serum tryptase | Measures event-related mast-cell activation | Timing is critical and a normal absolute value may still represent a significant rise from a low baseline |
| Baseline serum tryptase | Provides the comparator for the acute sample and may suggest mastocytosis or hereditary alpha-tryptasaemia | A raised baseline does not by itself diagnose MCAS or mastocytosis |
| Urinary N-methylhistamine | May provide indirect evidence of histamine release | Collection, diet, medicines and laboratory methods can affect interpretation |
| Urinary leukotriene E4 | May reflect cysteinyl-leukotriene production | It is not specific to MCAS and is not available universally |
| Urinary prostaglandin metabolites | May support prostaglandin D2 pathway activation | Assays and validated thresholds vary |
| Plasma histamine | Sometimes requested during reactions | Short half-life and difficult sample handling limit routine reliability |
| Allergy testing | Identifies a specific IgE-mediated trigger | A positive allergy test does not independently diagnose MCAS |
Chronic non-specific symptoms alone are insufficient
Fatigue, headache, brain fog, abdominal discomfort, palpitations and breathlessness occur in many conditions. Without systemic episodes and objective mediator evidence, these symptoms do not establish consensus MCAS.
Understanding tryptase testing
Acute tryptase
Timing
The sample should be obtained during or soon after a significant systemic reaction.
Purpose
It captures the temporary rise associated with mast-cell degranulation.
Limitation
A sample collected too late may miss the rise.
Baseline tryptase
Timing
It is measured after complete recovery from the episode.
Purpose
It provides the patient-specific reference required for the 20% plus 2 formula.
Raised result
Persistent elevation may prompt assessment for mastocytosis, hereditary alpha-tryptasaemia or another cause.
The 20% plus 2 rule
Formula
Significant acute value > baseline × 1.2 + 2 ng/mL.
Example
With a baseline of 5 ng/mL, an acute result above 8 ng/mL satisfies the formula.
Why it matters
It accounts for differences between patients with naturally low or high baseline tryptase.
MCAS, mastocytosis and hereditary alpha-tryptasaemia
These terms are related but are not interchangeable. A patient can have mast-cell activation symptoms without having an excessive number of mast cells.
| Condition | Underlying issue | Key diagnostic feature |
|---|---|---|
| Mast Cell Activation Syndrome | Recurrent inappropriate mediator release | Systemic episodes, objective mediator rise and treatment response |
| Systemic mastocytosis | Clonal accumulation of abnormal mast cells in bone marrow or other extracutaneous organs | WHO pathological, molecular, immunophenotypic and tryptase criteria |
| Hereditary alpha-tryptasaemia | Increased inherited copies of the alpha-tryptase-encoding gene | Persistently raised baseline tryptase with confirmatory genetic copy-number testing |
| Ordinary IgE-mediated allergy | Immune response to a defined allergen | Reproducible exposure history supported by selected allergy testing |
WHO diagnostic criteria for systemic mastocytosis
Major criterion
Multifocal dense aggregates of at least 15 tryptase-positive and/or CD117-positive mast cells in bone marrow or another extracutaneous organ.
Atypical morphology
More than 25% of mast cells are spindle-shaped, immature or otherwise atypical in the relevant marrow or tissue sample.
Activating KIT mutation
An activating KIT mutation at codon 816 or another critical KIT region is detected in bone marrow or another extracutaneous organ.
Aberrant surface markers
Mast cells abnormally express CD2, CD25 and/or CD30 in addition to normal mast-cell markers.
Persistently raised tryptase
Baseline serum tryptase exceeds 20 ng/mL, subject to recognised exclusions and adjustment where hereditary alpha-tryptasaemia is present.
A baseline tryptase above 20 ng/mL is not diagnostic by itself
Systemic mastocytosis usually requires the major criterion plus at least one minor criterion, or at least three minor criteria. Tryptase must be interpreted alongside pathology, genetics and immunophenotyping.
Treatment of Mast Cell Activation Syndrome
Treatment is individualised according to the dominant mediators, organ systems involved, trigger pattern, episode severity and any underlying clonal or allergic disorder.
H1 antihistamines
Non-sedating H1 blockers may reduce itching, hives, flushing, nasal symptoms and some histamine-related reactions.
H2 antihistamines
H2 blockers may help selected gastrointestinal or histamine- mediated symptoms.
Leukotriene antagonists
Medicines such as montelukast may help selected bronchial, nasal or leukotriene-mediated symptoms.
Mast-cell stabilisers
Cromoglicate or ketotifen may be considered by specialists for selected symptom patterns.
Asthma treatment
Confirmed asthma should be treated according to asthma guidance, including corticosteroid-containing inhalers where indicated.
Omalizumab
Anti-IgE treatment may be considered in selected patients with severe allergic disease, recurrent anaphylaxis or qualifying asthma.
Adrenaline
Intramuscular adrenaline is first-line treatment for anaphylaxis, not antihistamines or inhalers alone.
Psychological support
Support can help patients manage fear of reactions, treatment burden and the impact of unpredictable symptoms.
Antihistamines do not replace adrenaline in anaphylaxis
Antihistamines may improve skin symptoms but do not reliably treat airway swelling, severe bronchospasm or circulatory collapse. Follow the prescribed emergency plan and use an adrenaline auto-injector promptly when indicated.
Managing respiratory symptoms safely
| Symptom | Possible treatment pathway | Important qualification |
|---|---|---|
| Asthma-type wheeze | Reliever bronchodilator and corticosteroid-containing asthma therapy where asthma is confirmed | Repeated escalation without objective asthma evidence may miss another diagnosis |
| Nasal congestion | H1 antihistamine, saline irrigation or intranasal corticosteroid according to the underlying diagnosis | Persistent one-sided blockage, bleeding or severe pain needs ENT assessment |
| Throat tightness | Emergency adrenaline for suspected anaphylaxis; laryngeal assessment for recurrent non-emergency episodes | Do not assume every throat sensation represents airway swelling |
| Dysfunctional breathing | Respiratory physiotherapy, pacing and treatment of coexisting respiratory disease | Important cardiopulmonary causes must be assessed first |
| Persistent cough | Structured assessment for asthma, rhinitis, reflux, infection and other causes | Empirical mast-cell treatment alone may delay the correct diagnosis |
Lung physiology can be assessed through the lung-function testing service .
Identifying and managing triggers
Trigger patterns vary. Avoidance should be targeted and proportionate rather than based on an indefinitely expanding list of suspected exposures.
Defined allergens
Foods, medicines, insect venom, animals or pollens may trigger IgE-mediated reactions in sensitised patients.
Heat and temperature change
Hot environments, fever or abrupt temperature shifts may aggravate symptoms in some patients.
Exercise
Exercise may act as an independent trigger or amplify a food-dependent allergic reaction.
Alcohol
Alcohol can cause vasodilation, impair histamine metabolism and act as a cofactor during allergic reactions.
Medicines
NSAIDs, opioids, contrast agents or other medicines may trigger reactions through different mechanisms.
Infection and stress
Illness, sleep loss and physiological stress may lower the threshold for symptoms without being the primary disease cause.
Keep a structured episode diary
Record timing, foods, medicines, exercise, temperature, skin symptoms, gastrointestinal symptoms, breathing symptoms, blood pressure where available and treatment response. This is more useful than recording isolated symptoms without context.
Exercise with MCAS or suspected mast-cell symptoms
Physical activity supports cardiovascular fitness, respiratory function, mood and musculoskeletal health. The programme should be adjusted when exercise reliably triggers systemic reactions, bronchospasm or post-exertional symptoms.
Begin below the symptom threshold
Start with a duration and intensity that does not repeatedly provoke significant symptoms.
Warm up gradually
A progressive warm-up may reduce sudden bronchial, cardiovascular or temperature-related stress.
Control temperature
Exercise in a cooler environment when heat is a recognised trigger and maintain appropriate hydration.
Carry prescribed emergency medication
Patients at risk of anaphylaxis should have rapid access to their adrenaline auto-injectors and emergency plan.
Consider exercise-induced asthma
Cough or wheeze during activity may require objective assessment for exercise-induced bronchoconstriction.
Avoid exercising alone after severe reactions
A supervised or accompanied setting may be safer while triggers and emergency treatment are being clarified.
Stop exercising and use the emergency plan for:
- throat swelling or progressive throat tightness;
- widespread hives with breathing difficulty;
- wheeze that is rapidly worsening;
- dizziness, faintness or collapse;
- severe abdominal symptoms with respiratory symptoms;
- blue or grey lips or skin.
Diet and histamine
A restrictive low-histamine diet is not required for every person with suspected MCAS. Evidence is limited, food histamine content varies and excessive restriction can impair nutrition.
Some patients report reproducible symptoms after fermented foods, aged products, alcohol or foods stored for prolonged periods. Others tolerate these foods without difficulty.
Use a time-limited trial
A short, structured exclusion followed by careful reintroduction is more informative than indefinite avoidance.
Keep a food-and-symptom diary
Record portion, preparation, storage, cofactors and the timing of symptoms.
Avoid unnecessary restriction
Removing many foods can lead to weight loss, micronutrient deficiency and increased anxiety around eating.
Seek dietetic advice
A registered dietitian can maintain nutritional adequacy while investigating reproducible food-related symptoms.
Food intolerance is not the same as food allergy
Immediate hives, swelling, wheeze, vomiting or collapse after a particular food requires specialist allergy assessment. Do not rely on a low-histamine diet to prevent anaphylaxis.
When should urgent help be sought?
Arrange medical review when:
- breathing symptoms recur with flushing, hives or diarrhoea;
- asthma-type symptoms are not objectively confirmed;
- several episodes occur without an identified trigger;
- baseline tryptase is persistently raised;
- symptoms require repeated corticosteroids or emergency care;
- there is recurrent faintness or unexplained low blood pressure;
- an adrenaline auto-injector has been used;
- the diagnosis remains uncertain despite treatment.
Call 999 and use prescribed adrenaline for suspected anaphylaxis
- throat swelling or rapidly progressive throat tightness;
- difficulty breathing, severe wheeze or stridor;
- collapse, fainting or severe dizziness;
- blue or grey lips or skin;
- confusion or marked drowsiness;
- rapidly developing symptoms affecting more than one organ system;
- severe symptoms after food, medicine or insect sting exposure.
Lie flat with the legs raised when possible, unless breathing is easier in a supported sitting position. Do not stand or walk suddenly after adrenaline has been given.
Conclusion
Mast cells release mediators that can affect the nose, throat, bronchi, skin, gastrointestinal tract and cardiovascular system.
Wheeze, breathlessness, chest tightness, cough and nasal congestion can occur during systemic mast-cell activation, but none of these symptoms is specific to MCAS.
Asthma, inducible laryngeal obstruction, dysfunctional breathing, rhinosinusitis, reflux, infection and cardiac disease should be assessed according to the clinical pattern.
Consensus MCAS diagnosis requires recurrent systemic episodes involving at least two organ systems, objective evidence of event-related mediator release and improvement with mast-cell- directed treatment.
Acute and baseline tryptase testing should be interpreted using the patient-specific 20% plus 2 formula. A raised baseline tryptase alone does not prove MCAS or systemic mastocytosis.
Treatment may include antihistamines, leukotriene antagonists, selected mast-cell stabilisers, treatment of confirmed asthma and an emergency adrenaline plan where anaphylaxis risk is present.
Patients with recurrent wheeze, cough, throat symptoms or breathlessness can arrange a specialist respiratory assessment to identify whether a pulmonary or upper-airway condition is also contributing.
Frequently asked questions
Can MCAS cause wheezing?
Mast-cell mediators can contribute to bronchoconstriction and wheeze, especially when asthma coexists. Wheeze should still be assessed objectively.
Can MCAS cause breathlessness without asthma?
Yes, breathlessness may occur during systemic reactions, upper-airway symptoms or low blood pressure, but other respiratory and cardiac causes must be assessed.
Is chronic cough a typical sign of MCAS?
Persistent isolated cough is not specific to MCAS. Asthma, rhinitis, reflux, infection and other causes are usually more common.
Does throat tightness always mean anaphylaxis?
No. It may also reflect inducible laryngeal obstruction, reflux or dysfunctional breathing. Rapidly progressive tightness with swelling, wheeze or collapse should be treated as an emergency.
What are the main diagnostic criteria for MCAS?
Recurrent systemic episodes affecting at least two organ systems, objective mediator elevation and improvement with mast-cell-directed treatment.
What is the tryptase 20% plus 2 rule?
The acute tryptase should exceed the baseline multiplied by 1.2, plus 2 ng/mL, to support significant systemic mast-cell activation.
Can MCAS occur with a normal baseline tryptase?
Yes. The relevant evidence is an event-related rise above the individual baseline, not simply an absolute value above the laboratory reference range.
Does raised tryptase mean mastocytosis?
No. Hereditary alpha-tryptasaemia, kidney disease and other conditions can raise baseline tryptase. Mastocytosis requires additional WHO criteria.
Is MCAS the same as mastocytosis?
No. MCAS concerns mediator release. Mastocytosis is a clonal disorder involving abnormal accumulation of mast cells.
Can MCAS weaken the immune system?
MCAS is not generally classified as an immune-deficiency disorder. Recurrent bacterial infection may require a separate immune and respiratory investigation.
Are respiratory infections more severe in MCAS?
Infection may aggravate asthma, rhinitis or mast-cell symptoms, but evidence does not show that MCAS alone routinely causes severe respiratory infection.
Do antihistamines treat asthma?
Antihistamines can help allergic rhinitis and some mast-cell symptoms but do not replace corticosteroid-containing inhalers when asthma is confirmed.
Can montelukast help respiratory symptoms?
It may help selected patients with asthma, allergic rhinitis or leukotriene-mediated symptoms, but risks and benefits should be reviewed individually.
Is a low-histamine diet essential?
No. Some patients report benefit, but evidence is limited and tolerance varies. Avoid excessive restriction without dietetic guidance.
Can exercise trigger mast-cell symptoms?
Exercise, heat or a food-exercise combination can trigger symptoms in selected people. A personalised safety plan is important after severe reactions.
When should adrenaline be used?
Use prescribed intramuscular adrenaline promptly when the emergency plan indicates suspected anaphylaxis, especially with breathing difficulty, throat swelling, collapse or rapidly progressive multi-system symptoms.
References and further information
- Weiler CR, Austen KF, Akin C, et al. AAAAI Mast Cell Disorders Committee Work Group Report: Mast cell activation syndrome diagnosis and management. View the AAAAI work-group report
- World Health Organization and International Consensus Classification. Updated diagnostic criteria for systemic mastocytosis. View the comparative diagnostic-criteria table
- National Institute for Health and Care Excellence. Asthma: diagnosis, monitoring and chronic asthma management. NICE guideline NG245. View the NICE asthma guideline
- London Chest Specialist. Asthma diagnosis and treatment. Read about specialist asthma assessment
- London Chest Specialist. Chronic-cough specialist assessment. Read about persistent-cough investigation
- London Chest Specialist. Breathlessness diagnosis and treatment. Read about specialist breathlessness assessment
- London Chest Specialist. Fractional exhaled nitric oxide: a quick guide. Read the FeNO testing guide
- London Chest Specialist. Breathing-pattern dysfunction. Read about dysfunctional breathing
- London Chest Specialist. Lung-function tests. Read about spirometry and respiratory physiology
- London Chest Specialist. Chest-infection diagnosis and treatment. Read about recurrent respiratory infections
