Long COVID, Spike Protein and MCAS: What the Research Shows
Most people recover from COVID-19 within a few weeks. Some do not. They are left with fatigue, brain fog, palpitations, dizziness, digestive symptoms, disrupted sleep and exercise intolerance, the core features of Long COVID.
In clinic we also see a pattern that clients often describe in almost the same words: "I was fine with food, alcohol and supplements before COVID, and now I react to everything." Flushing, itching, headaches, racing heart, bloating and sensitivity to heat, fragrance or medications can look very much like Mast Cell Activation Syndrome (MCAS).
This raises the question. Could spike protein that lingers after infection be keeping the immune system, and mast cells in particular, switched on?
Does spike protein persist after COVID-19?
In some people, yes. Swank and colleagues used an ultrasensitive assay to detect circulating spike protein in people with post-acute COVID-19 sequelae up to 12 months after infection. A later study by Peluso and colleagues found SARS-CoV-2 antigens, most often spike, in a minority of plasma samples up to 14 months after infection. Tissue work points the same way. A 2023 review in Nature Immunology assembled evidence of viral RNA and protein persisting in tissues, including the gut, for months after acute illness.
The gut is where the mast cell question becomes most interesting. In a controlled study of people two years after mild COVID-19, terminal ileum biopsies from those with post-COVID syndrome showed persistent viral nucleocapsid protein. A 2026 preprint of colon and ileum biopsies found spike transcript and protein in tissue, and spike-positive areas in people with Long COVID showed a distinct inflammatory signature, with more macrophages, monocytes and plasma cells. . That was a small study (8 patients, 5 controls), so it is hypothesis-generating rather than definitive.
There are real caveats. Detection rates vary between cohorts, and spike has also been found in some people who recovered fully. A recent blinded longitudinal study found circulating antigen had become undetectable in both Long COVID and comparison groups by two years. Finding viral material also does not tell us whether it is doing anything. Fragments of protein are not the same as replicating virus, and blood levels may not reflect what sits in tissue.
Is there evidence of mast cell activation in Long COVID?
There is a growing body of evidence.
Afrin and colleagues proposed in 2020 that COVID-19 hyperinflammation and post-COVID illness might be rooted in mast cell activation. Weinstock and colleagues then found that people with Long COVID reported mast cell activation symptoms at a burden similar to people with diagnosed MCAS. This was symptom-based, so it shows overlap rather than confirming MCAS.
Treatment studies are suggestive but small. Glynne and colleagues reported T cell alterations and symptomatic response to antihistamines in Long COVID after mild infection. Salvucci and colleagues treated 14 patients with fexofenadine 180 mg and famotidine 40 mg daily, and compared them with 13 untreated patients. Fatigue, brain fog, abdominal symptoms and heart rate improved over 20 days. With no placebo arm and such small numbers, this supports the hypothesis.
The most objective data come from the ileum study mentioned above. Two years after infection, people with post-COVID syndrome had more activated mast cells in ileal tissue and higher serum tryptase and zonulin than recovered people without symptoms. Raised zonulin points towards impaired gut barrier function, which fits the wider picture.
One point matters clinically. Most of this literature describes mast cell activation or MCAS-like symptoms, and few studies apply formal MCAS diagnostic criteria. Those criteria require episodic symptoms in at least two organ systems, a measurable rise in mediators such as tryptase during a reaction, and a response to mast cell directed therapy. Not everyone with post-COVID reactivity will meet them.
Long COVID and MCAS: Is There Evidence of Mast Cell Activation?
In the laboratory, yes. Tsilioni and Theoharides exposed cultured human mast cells to full-length recombinant spike protein at 1 to 10 ng/mL. It triggered release of tryptase, chymase and IL-1β, and IL-33 amplified the effect. The protease release appeared to work through ACE2 and the IL-1β release through TLR4. The receptor-binding domain alone did not reproduce the IL-1β response.
This gives a credible mechanism, particularly in ACE2-rich, mast cell-dense tissue such as the gut. It remains an in vitro finding. It does not show that circulating or tissue spike protein activates mast cells in a living patient at clinically relevant concentrations.
Why do people become so reactive after COVID-19?
Mast cells store histamine, tryptase, prostaglandins and leukotrienes, and release cytokines when triggered. If the threshold for activation drops, ordinary triggers such as food, alcohol, heat, exercise, stress and fragrance can produce a disproportionate response. Some people find that a temporary low-histamine diet or mast cell stabilisation eases symptoms, although histamine is rarely the whole explanation.
In most patients this is probably a multi-factorial picture. Persistent antigen may be one input. Others include gut dysbiosis, SIBO and increased intestinal permeability, autonomic dysfunction, hormonal shifts, environmental exposures such as mould, and a pre-existing tendency towards mast cell reactivity. The nervous system is part of this too. Mast cells and neurones signal to each other, and prolonged physiological stress can make mast cells easier to trigger.
It also helps to keep the differential open. Post-COVID reactivity can overlap with POTS, ME/CFS, autoimmune disease, iron deficiency, thyroid dysfunction, sleep disorders, medication effects and chronic environmental illness.
Testing for spike protein
Where persistent spike is a consideration, we can arrange testing through Magdeburg Molecular Diagnostics (MMD) in Germany. Spike can sit in different compartments, so the panel looks at plasma, immune cells and exosomes, alongside the antibody response.
SSP1, SARS-CoV-2 spike protein in plasma/serum. A high-sensitivity quantitative ELISA with a detection limit of 4.5 pg/mL. It measures free spike only, so spike bound to antibody is not detected.
SSP2, SARS-CoV-2 spike protein in immune cells (PBMCs). Measures spike taken up by circulating immune cells such as macrophages.
SSP3, SARS-CoV-2 spike protein in exosomes. Measures spike carried within exosomes purified from around 4 mL of plasma or serum.
SSP5, IgG antibodies to SARS-CoV-2 (S protein). Measures the antibody response to spike after infection or vaccination.
The rationale for testing more than one compartment is that spike is mostly found in ACE2-rich tissue and is not always easy to catch in the bloodstream. Immune cells can take it up, which makes it measurable in a small fraction of circulating cells, and exosomes may carry it around the body. The lab recommends interpreting SSP2 and SSP3 together, and notes that a negative PBMC result does not rule out persistent spike. It suggests plasma, serum or exosome testing if suspicion remains.
These tests also have limits. A negative result does not exclude tissue persistence either, because blood levels may not reflect what is happening in the gut or other tissues.
For that reason we treat these results as one piece of information. They are read alongside symptoms, history and other markers, including mast cell mediators, gut assessment and autonomic findings.
Long COVID, Spike Protein and MCAS: What This Means in Practice
Long COVID is unlikely to have a single cause, and spike protein is best seen as one candidate mechanism among several, but a factor to consider. The more useful clinical question is why this person's immune system remains activated and what is keeping their mast cells reactive.
That usually means a careful history and targeted testing. We carefully look at the client's timeline, consider gut function, autonomic symptoms, nutrient status, hormones and environmental exposures, then build a plan step by step.
At The Autoimmune Clinic we work with Long COVID, MCAS, histamine intolerance, POTS, gut dysfunction and mould-related illness. If you have developed food and medication reactions, flushing, palpitations or digestive symptoms since COVID-19, we can help you work out what is driving them and what to do next.
