Vitamin D and the Immune System
Vitamin D receptors (VDR) are expressed on nearly every immune cell type — including T cells, B cells, dendritic cells, macrophages, and neutrophils. The active metabolite 1,25-dihydroxyvitamin D acts as an immunomodulator, tuning both the innate response to pathogens and the tolerance mechanisms that limit autoimmunity.
Innate immunity: cathelicidin and defensins
When toll-like receptor 2 on macrophages recognises a pathogen (e.g., Mycobacterium tuberculosis), it upregulates CYP27B1 and VDR. Local 25(OH)D is converted to the active 1,25-dihydroxyvitamin D, which then drives transcription of cathelicidin (LL-37) and β-defensin-2 — antimicrobial peptides that puncture pathogen membranes. Individuals with low 25(OH)D produce less cathelicidin per macrophage stimulation (Liu et al. 2006).
Adaptive immunity: T-cell modulation
1,25-dihydroxyvitamin D biases the T-helper response away from pro-inflammatory Th1/Th17 profiles toward Th2 and regulatory T cells (Treg). It also inhibits dendritic-cell maturation, reducing antigen presentation. The net effect is dampened autoimmunity — a mechanism consistent with epidemiological links between low vitamin D and multiple sclerosis, type 1 diabetes, and inflammatory bowel disease.
What the randomised trials show
- Acute respiratory infections: Martineau et al.'s 2017 individual-patient-data meta-analysis (25 trials, ~11,000 participants) found supplementation reduced infection risk by ~12% overall, with a much larger effect (~70% reduction) in participants with baseline 25(OH)D below 10 ng/mL.
- Multiple sclerosis: no randomised trial has yet shown that supplementation prevents MS onset, but higher 25(OH)D in adolescence is consistently associated with lower adult risk in cohort studies.
- COVID-19: mixed. Observational data show low 25(OH)D correlates with worse outcomes, but large randomised trials (VIVID, CORONAVIT) have not shown clear benefit from supplementation in unselected populations. Benefit may be limited to deficient patients.
- Autoimmune disease incidence: VITAL post-hoc analysis showed a modest ~22% reduction in incident autoimmune disease with 2,000 IU/day over 5 years — hypothesis generating but not definitive.
Practical implications
The consistent message is that correcting deficiency yields the clearest immunological benefit. Supplementing a person who is already sufficient (≥ 30 ng/mL) is unlikely to further improve infection resistance. If you or your children get frequent respiratory infections and haven't measured your 25(OH)D, it's a reasonable test to request.