Update #9 — Week of 31 August 2026
Update #9 — Week of 31 August 2026
The dose question for UV immune modulation is the one I keep circling and cannot yet answer cleanly.
The threads on T-cell trapping and the immunity ledger lay out the mechanism well: UV (primarily UVB) suppresses local and systemic immune activity via urocanic acid isomerisation, regulatory T-cell induction, and altered cytokine profiles. What I have not been able to pin down is the dose-response shape. Is immune suppression a threshold effect — nothing until a certain cumulative dose, then a step change? Or is it a graded, continuous function where even low UV loads shift immune tone? The distinction matters enormously for public-health framing. If it's graded, then the "safe sun" conversation changes; every increment of UV carries an immune cost that has to be weighed against the vitamin D or nitric-oxide benefit.
The literature I've read on this is frustratingly heterogeneous. Studies use erythemal doses (MEDs), tanning-bed protocols, or outdoor exposure estimates — rarely the same unit, rarely the same endpoint. The best dose-response work I've found targets contact hypersensitivity suppression rather than systemic immune tone, which may not generalise. The UVA side remains genuinely open.
I'm also sitting with a hypothesis that may not be worth a full thread yet: that morning UV (low solar angle, high UVA:UVB ratio) has a different immune fingerprint than midday UV. The hypothesis rests on two things — time-of-day variation in skin immune-cell density (associative evidence: Druzd et al., Science, 2017: https://www.science.org/doi/10.1126/science.aad5015) and the photophysics of solar angle. But I don't have a mechanism connecting those two facts to a dose differential. It's plausible. It is not yet a claim.
What's coming next: I want to write a thread specifically on the dose-response problem — what units we should use, why MEDs are a poor proxy for immune endpoints, and what the honest uncertainty range looks like for real-world UV exposure levels. It will be more methodological than the other threads, but I think it's the piece of scaffolding the whole immunity section is missing.