Long COVID and MCAS: Why So Many Patients Have Both

Mast cell activation syndrome (MCAS) has emerged as one of the most significant comorbidities in Long COVID. Surveys of Long COVID patient communities consistently find that 30–50% of respondents report symptoms consistent with MCAS — flushing, hives, itching, gastrointestinal cramping, anaphylactoid reactions, and worsening of symptoms after eating, heat, or stress. For many patients, MCAS symptoms were not present before COVID-19 and appear to have been triggered or unmasked by the infection.

What Is MCAS?

Mast cells are immune cells found throughout the body — particularly in the skin, gut, lungs, and connective tissue — that act as first responders to perceived threats. In MCAS, mast cells degranulate inappropriately or excessively, releasing a flood of mediators including histamine, tryptase, prostaglandins, leukotrienes, and cytokines. These mediators cause the wide range of symptoms associated with MCAS.

MCAS is distinct from classic allergies (IgE-mediated) in that reactions can be triggered by non-immunological stimuli — heat, cold, vibration, stress, exercise, certain foods, medications, and infections. This makes MCAS particularly challenging to diagnose and manage.

Why Does COVID-19 Trigger MCAS?

Several mechanisms have been proposed to explain why SARS-CoV-2 infection triggers or worsens MCAS:

Direct mast cell activation. SARS-CoV-2 spike protein can directly activate mast cells via ACE2 receptors, CD147, and toll-like receptors. This direct activation may prime mast cells for subsequent hyperreactivity.

Persistent spike protein. As discussed in the viral persistence article, spike protein can circulate in the blood of Long COVID patients for months. Persistent spike protein exposure may continuously stimulate mast cells, maintaining a state of chronic low-level activation.

IgE sensitization. Some researchers have proposed that COVID-19 infection can trigger IgE sensitization to viral antigens, leading to IgE-mediated mast cell activation upon subsequent exposures or in response to cross-reactive antigens.

Autonomic dysfunction. The autonomic nervous system regulates mast cell activity. POTS and other forms of dysautonomia — which are extremely common in Long COVID — can directly trigger mast cell degranulation through adrenergic and cholinergic pathways. This creates a bidirectional relationship: MCAS worsens dysautonomia, and dysautonomia worsens MCAS.

Microbiome disruption. The gut microbiome plays a critical role in regulating mast cell activity. The profound microbiome disruption seen in Long COVID may remove key regulatory signals that normally keep mast cells in check.

Recognizing MCAS in Long COVID

The challenge with MCAS in Long COVID is that many of its symptoms overlap with other Long COVID manifestations — fatigue, brain fog, gastrointestinal symptoms, and cardiovascular symptoms can all be attributed to multiple mechanisms. However, certain patterns are more suggestive of a mast cell component:

• Symptoms that worsen after eating (particularly high-histamine foods)
• Flushing, hives, or itching without an obvious allergic trigger
• Anaphylactoid reactions (throat tightening, difficulty breathing, severe flushing) without IgE-mediated allergy
• Worsening with heat, exercise, or stress
• Multiple chemical sensitivities (fragrances, cleaning products, exhaust)
• Significant improvement with antihistamines

Diagnosis

MCAS diagnosis in Long COVID follows the same criteria as primary MCAS: episodic symptoms consistent with mast cell mediator release, laboratory evidence of elevated mast cell mediators during or shortly after a symptomatic episode, and response to mast cell-targeted therapy.

The most accessible laboratory test is serum tryptase, measured during or within 4 hours of a symptomatic episode. A value more than 20% above baseline (or above 11.4 ng/mL) is considered supportive. However, tryptase is often normal in MCAS — particularly in the non-clonal (secondary) form common in Long COVID — and normal tryptase does not rule out MCAS.

Additional tests include 24-hour urine histamine, N-methylhistamine, prostaglandin D2, and leukotriene E4. These require specialized laboratory processing and are not available at all centers.

Treatment Approach

Treatment of MCAS in Long COVID follows the same stepwise approach as primary MCAS, with some Long COVID-specific considerations.

First-line: H1 and H2 antihistamines. Cetirizine or loratadine (H1) combined with famotidine (H2) is the standard starting point. Many Long COVID-MCAS patients require higher doses than typical allergy patients — cetirizine 10 mg twice daily and famotidine 20–40 mg twice daily are commonly used.

Second-line: Mast cell stabilizers. Cromolyn sodium (oral, for gut symptoms) and ketotifen (systemic mast cell stabilizer with H1 antihistamine properties) are the most commonly used. Quercetin, a natural flavonoid with mast cell stabilizing properties, is widely used by patients and has some supporting evidence.

Third-line: Montelukast. The leukotriene receptor antagonist montelukast addresses the prostaglandin/leukotriene arm of mast cell activation that antihistamines do not cover.

Low-histamine diet. Reducing dietary histamine load can significantly reduce the total mast cell mediator burden. This involves avoiding aged cheeses, fermented foods, alcohol, canned fish, and other high-histamine foods.

Addressing the underlying Long COVID. Treatments targeting viral persistence, immune dysregulation, and autonomic dysfunction may reduce the underlying drive for mast cell activation. LDN, in particular, has mast cell-stabilizing properties and is used by many Long COVID-MCAS patients.

Key Takeaways

MCAS is a common and often underrecognized comorbidity in Long COVID. Its symptoms overlap significantly with other Long COVID manifestations, but targeted treatment with antihistamines and mast cell stabilizers can provide meaningful relief for many patients. Addressing MCAS is also important for managing POTS and other dysautonomia symptoms, given the bidirectional relationship between mast cell activation and autonomic dysfunction.

This article is for informational purposes only and does not constitute medical advice.