Mast cells: the sentinels of the immune system
Mast cells are long-lived tissue-resident immune cells found in high concentrations in the gut, skin, respiratory tract, and connective tissue — the surfaces through which the body interfaces with the external world. They are among the first responders to perceived threats, capable of rapidly releasing a complex mixture of preformed and newly synthesised mediators that initiate and modulate the immune response. Histamine, heparin, prostaglandins, leukotrienes, cytokines, proteases — a fully activated mast cell can release more than 60 distinct mediators within minutes.
In mast cell activation syndrome (MCAS), mast cells are triggered inappropriately or disproportionately — by stimuli that should not trigger a significant immune response, or with a magnitude of response that far exceeds what the trigger warrants. The result is a cyclical or episodic release of mediators that affects multiple organ systems simultaneously. Symptoms arise in the gut, the skin, the cardiovascular system, the respiratory system, and the nervous system, often in seemingly unrelated combinations that are puzzling to practitioners who are not looking for a unifying mechanism.
"MCAS is not rare. It is simply rarely considered. A patient who has been told their symptoms are psychosomatic, or who carries multiple diagnoses across multiple specialties with no unifying explanation, should have MCAS investigated as a potential mechanism."
The MCAS symptom pattern
What makes MCAS diagnostically challenging is precisely the breadth and apparent inconsistency of its manifestations. A patient with MCAS might experience unpredictable flushing and skin reactions alongside gut cramping, brain fog, joint pain, palpitations, and extreme fatigue — all within the same episode, triggered by something as apparently innocuous as a particular food, a temperature change, physical exertion, emotional stress, or a specific smell. The pattern does not fit neatly into any standard diagnostic category.
This diagnostic complexity is compounded by the variability of MCAS — symptoms can fluctuate significantly between episodes, and the triggers are often cumulative rather than individually sufficient. The threshold model of mast cell activation is clinically important: the total burden of triggers at any given time determines whether the threshold for activation is crossed, meaning that the same food or stressor may provoke symptoms on one day and not another, depending on what else is contributing to the total load.
Gut symptoms
Nausea, abdominal cramping, bloating, diarrhoea, and reflux — particularly episodic and not consistently tied to any single food.
Skin manifestations
Flushing, hives, dermatographia (whealing with light pressure), itching, and unexplained rashes — often episodic and disproportionate to apparent trigger.
Cardiovascular
Palpitations, presyncope, orthostatic hypotension, and unexplained episodes of low blood pressure, often associated with the postural tachycardia of POTS, which frequently co-occurs with MCAS.
Neurological
Cognitive difficulties, anxiety, brain fog, and neuropathic pain — mediated by the direct effects of histamine and other mast cell mediators on neuronal function.
Hormonal interactions
Oestrogen activates mast cells. Progesterone and testosterone have a modulating effect. Hormonal fluctuations in the menstrual cycle significantly affect MCAS symptom severity in many patients.
Connective tissue overlap
A significant proportion of MCAS patients carry a concurrent diagnosis of hypermobile Ehlers-Danlos syndrome (hEDS). The triad of MCAS, POTS, and hEDS is now well-recognised in the literature as a frequently co-occurring cluster.
Investigation and management
Diagnosing MCAS requires demonstrating elevated mast cell mediators — serum tryptase, prostaglandin D2, histamine, and urinary N-methylhistamine and 11β-prostaglandin F2α — ideally timed to a symptomatic episode. Tryptase alone, which is the standard mast cell marker ordered in emergency settings, is often normal in MCAS, as the tryptase-rich granules are less commonly released in non-anaphylactic presentations. A normal tryptase does not exclude MCAS.
Management is multifactorial and requires a coordinated approach. Mast cell stabilisers, antihistamines (H1 and H2 blockers), and low-dose aspirin form the pharmacological backbone for many patients. Trigger identification and avoidance — including dietary modification, environmental adjustment, and stress management — is critically important and requires careful, systematic tracking. Addressing the underlying factors that increase mast cell reactivity — gut dysbiosis, hormonal imbalance, chronic stress, nutrient deficiencies — requires a breadth of clinical oversight that no single specialty provides.
MCAS is, perhaps more than any condition in this journal, a condition that demands coordination. Its multi-system nature, its complex trigger landscape, its overlap with other conditions, and its sensitivity to the total physiological load mean that it cannot be managed effectively in silos. Every aspect of a patient's physiology interacts with every other aspect. Managing it well requires someone to hold the whole picture — continuously, across all the practitioners involved.