What is a mast cell, and what is mast cell activation syndrome (MCAS)?
Mast cells are immune cells that live in tissues. MCAS refers to systemic episodes of mast cell activation that meet specific criteria.
- 12 sources
- 4 min read
- Clinical review
- Verified 9 November 2026
Key points
- Mast cells are tissue-resident immune effector cells; they are thought to play a central role in allergic inflammatory diseases and they interact with other immune cells, neurons and the structural cells of tissues [1, 2].
- They can release many mediators, including histamine, tryptase, prostaglandins, leukotrienes and cytokines. Mast cell activation is a biological mechanism, not in itself a diagnosis of MCAS [3, 2, 4].
- The ECNM-AIM consensus framework requires severe, recurrent systemic episodes involving at least two organ systems, biochemical evidence of mast cell activation, and a response to treatment that targets the mediators or their action [3, 4, 5].
- Mast cell activation disorders (MCAD) form a broader category that includes IgE-mediated allergies, clonal mast cell diseases such as systemic mastocytosis, and MCAS; their criteria and mechanisms are not interchangeable [6].
Mast cell biology
Mast cell activation, mast cell disease and MCAS: three different concepts
Clinical presentations described
In an emergency. If you have trouble breathing, throat swelling or feel faint after an exposure, call 911 (or your local emergency number); never delay emergency care for a blood test.
What is still unknown
Institutional reference points
References and level of evidence
- Pahima HT, Dwyer DF. Update on mast cell biology. J Allergy Clin Immunol, 155(4):1115–1123 2025. DOI: 10.1016/j.jaci.2024.12.1092. PMID: 39800266.(evidence synthesis)Limitations: Biology review (2022–2024 studies, mouse and human), not a diagnostic guideline. Read: abstract.Declared conflicts of interest: Not reviewed in the part of the source that was read
- Valent P, Bonadonna P, Hartmann K, et al. Why the 20% + 2 Tryptase Formula Is a Diagnostic Gold Standard for Severe Systemic Mast Cell Activation and Mast Cell Activation Syndrome. Int Arch Allergy Immunol, 180(1):44–51 2019. DOI: 10.1159/000501079. PMID: 31256161.(evidence synthesis)Limitations: Focused narrative review written by the group that devised the formula; not an original validation study. Read: full text (author manuscript PMC7115850).Declared conflicts of interest: Several authors declare research funding, consulting or fees (including Deciphera, Novartis, Blueprint, Thermo Fisher/Phadia); others declare none
- Valent P, Akin C, Arock M, et al. Definitions, criteria and global classification of mast cell disorders with special reference to mast cell activation syndromes: a consensus proposal. Int Arch Allergy Immunol, 157(3):215–225 2012. DOI: 10.1159/000328760. PMID: 22041891.(guideline or consensus)Limitations: Consensus proposal from an expert conference (2010), refined by later updates; not a population study. Read: full text (PMC3224511).Declared conflicts of interest: No declaration found in the text that was read
- Valent P, Hartmann K, Bonadonna P, Niedoszytko M, Triggiani M, Arock M, Brockow K. Mast Cell Activation Syndromes: Collegium Internationale Allergologicum Update 2022. Int Arch Allergy Immunol, 183(7):693–705 2022. DOI: 10.1159/000524532. PMID: 35605594.(guideline or consensus)Limitations: Expert synthesis (Collegium Internationale Allergologicum), not a meta-analysis. Read: full text (PMC9393812).Declared conflicts of interest: Several authors declare consulting activities, fees or research funding (including Novartis, Blueprint, Deciphera, Thermo Fisher); P. Valent funded by the Austrian Science Fund (FWF)
- Lee E, Picard M. Diagnosis and management of mast cell activation syndrome (MCAS) in Canada: a practical approach. Allergy Asthma Clin Immunol, 21(1):49 2025. DOI: 10.1186/s13223-025-00998-9. PMID: 41272881.(evidence synthesis)Limitations: Practical narrative review based on the ECNM-AIM criteria, adapted to the Canadian setting; test availability varies by country. Read: full text (PMC12639879).Declared conflicts of interest: M. Picard: fees from Novartis, Sanofi, Lundbeck and Celltrion unrelated to this work, associate editor of the journal, investigator in a study funded by Telios; E. Lee: none
- Akin C, Gülen T, Castells MC, Oude Elberink H, Valent P. Diagnosis and Management of Patients With Mast Cell Activation Syndromes: Status 2026. J Allergy Clin Immunol Pract, 14(1):19–28 2026. DOI: 10.1016/j.jaip.2025.10.046. PMID: 41285202.(expert opinion)Limitations: Expert clinical review, not an original study. Read: abstract.Declared conflicts of interest: Not reviewed in the part of the source that was read
- Afrin LB, Blitshteyn S, Bluestein LS, Dempsey TT, Maxwell AJ, Miller CS, Nagy È, Nugent DJ, Schofield JR, Weinstock LB, Xi SC, Molderings GJ. Progress in mast cell activation syndrome: the global consensus-2 diagnostic criteria at six years. Diagnosis (Berl) 2026. DOI: 10.1515/dx-2026-0016. PMID: 42240520.(expert opinion)Limitations: Review written by authors who promote the "consensus-2" framework; it is not a professional society guideline. Read: abstract (full text not consulted).Declared conflicts of interest: Not reviewed in the part of the source that was read
- Castells M, Giannetti MP, Hamilton MJ, et al. Mast cell activation syndrome: Current understanding and research needs. J Allergy Clin Immunol, 154(2):255–263 2024. DOI: 10.1016/j.jaci.2024.05.025. PMID: 38851398.(evidence synthesis)Limitations: Narrative review of knowledge gaps and research needs. Read: abstract (full text not accessible from the verification environment).Declared conflicts of interest: Not reviewed in the part of the source that was read
- Giavina-Bianchi P, Giavina-Bianchi M, Morais-Almeida M, et al. Mast cell activation syndrome: Global perspectives from a World Allergy Organization survey on knowledge, diagnosis, and access to care. World Allergy Organ J, 19(10):101470 2026. DOI: 10.1016/j.waojou.2026.101470. PMID: 42857128.(observational study)Limitations: Self-reported cross-sectional survey (547 allergists/immunologists analyzed, responses collected March to May 2025); possible response bias; describes respondents' practices, not the prevalence of MCAS. Read: full text (PMC13651491).Declared conflicts of interest: The authors declare no financial conflicts of interest; funding: not applicable
- French mastocytosis reference center (CEREMAST), MaRIH rare-disease network; coordinated by Dr Cristina Livideanu. Mastocytoses non-avancées chez l'adulte — Protocole national de diagnostic et de soins (PNDS). French national diagnosis and care protocol (PNDS), June 2022 (published on the website of the French National Authority for Health, HAS) 2022.(guideline or consensus)Limitations: Covers non-advanced mastocytosis in adults; it is not a stand-alone PNDS devoted to isolated MCAS. Read: table of contents, scope and selected passages.Declared conflicts of interest: Not reviewed in the part of the source that was read
- French mastocytosis reference center (CEREMAST), MaRIH rare-disease network. Mastocytoses avancées de l'adulte — PNDS. HAS (French National Authority for Health), institutional web page, posted July 17, 2025 2025.(guideline or consensus)Limitations: Scope limited to advanced mastocytosis; HAS states that it has not validated this PNDS. Read: HAS presentation page (PNDS document not read).Declared conflicts of interest: Not reviewed in the part of the source that was read
- UK Parliament, House of Commons; answer from the Department of Health and Social Care. Mast Cell Activation Syndrome: Health Services — written question UIN 803, answered 30 June 2026. UK Parliament, Written questions and answers 2026.(expert opinion)Limitations: Written ministerial answer, not a clinical guideline; describes the situation as of June 30, 2026.Declared conflicts of interest: Not applicable