Morcos & Theoharides 2026 — Mast cells in long COVID neuropathy

One-paragraph summary

Narrative review by Theoharides (a foundational researcher in mast-cell biology and chronic-pain syndromes) and Morcos arguing that mast cell (MC) activation is a central contributor to long-COVID (PASC) neuropathic manifestations. Mechanistic claims: (1) SARS-CoV-2 spike protein activates MCs via ACE2 and TLR4; (2) activated MCs release histamine, tryptase, IL-1β, IL-6, TNFα; (3) these mediators sensitize peripheral nerves, disrupt the BBB, and recruit microglia; (4) the resulting MC-activation signature mirrors patterns seen in small-fiber neuropathy and ME/CFS, suggesting a shared immune-mediated etiology across PASC, FM, and ME/CFS. The review also notes early evidence for MC-directed treatments (mast-cell stabilizers, antihistamines, leukotriene antagonists) with variable response. Adds mast_cell and histamine to the project ontology.

Claims as triples

• mast_cell — causes → small_fiber_neuropathy [evidence: review of MC-SFN mechanism literature; confidence: emerging]
• mast_cell — causes → neuroinflammation_glial [evidence: MC mediators recruit microglia and disrupt BBB; confidence: emerging]
• histamine — modulates → small_fiber_neuropathy [confidence: emerging]
• mast_cell — bridges → post_covid_syndrome [confidence: bridging]

Methods note

Narrative review, not systematic. Theoharides is a recognized authority on MC biology in chronic-pain syndromes; the review consolidates evidence from his own and adjacent groups across post-viral pain literature.

Limitations

• Narrative review; selection bias possible.
• The MC-as-central-mediator framing is partly Theoharides's long-standing hypothesis — the review is internally consistent but not an independent assessment.
• "MC patterns mirror SFN/ME-CFS" is a phenotype-level claim; molecular cross-validation is incomplete.
• MC-directed treatment evidence is described as "variable" — not strong enough to confidently recommend therapeutic deployment.

Open questions raised

• Are anti-SGC IgG and mast-cell activation the same patients or separate subtypes of FM/long-COVID? Critical question for subtype mapping.
• Do mast-cell stabilizers (cromolyn, ketotifen) reduce anti-SGC IgG titers, or do they act independently of the autoantibody mechanism?
• Could mast cells be the upstream driver of anti-SGC IgG production? (MC → B-cell signaling is established in other autoimmune contexts.)

Triangulation notes

• Strengthens bridge B1 (Long COVID ↔ FM via autoantibody-mediated SFN) by adding a candidate shared cellular mechanism (mast cells) on top of the antibody-mediated mechanism.
• Strengthens bridge B4 (ME/CFS ↔ FM via post-viral neuroimmune): mast cell signature links the three conditions on the immune-cell axis.
• Connects to B6 (predictive-coding / interoceptive-inference failure): MC activation could be one of the post-infectious neuroimmune perturbations Strube 2026's framework subsumes as the upstream driver of "central overload."
• Therapeutic implication: mast-cell-directed therapy (cromolyn, ketotifen, low-histamine diet, leukotriene antagonists) becomes a parallel cure-path candidate to the BA-IgG arm. Worth adding as a stage-2 alternative in the staged research program.