I was going to do an entry on Vitamin D, but the Wikipedia article is very extensive and worth reading.
immunological effects by binding to nuclear vitamin D receptors (VDR) which are present in most immune cell types including both innate and adaptive immune cells. The VDR is expressed constitutively in monocytes and in activated macrophages, dendritic cells, NK cells, T and B cells. In line with this observation, activation of the VDR has potent anti-proliferative, pro-differentiative, and immunomodulatory functions including both immune-enhancing and immunosuppressive effects.
VDR ligands have been shown to increase the activity of natural killer cells, and enhance the phagocytic activity of macrophages. Active vitamin D hormone also increases the production of cathelicidin, an antimicrobial peptide that is produced in macrophages triggered by bacteria, viruses, and fungi. Vitamin D deficiency tends to increase the risk of infections, such as influenza and tuberculosis. In a 1997 study, Ethiopian children with rickets were 13 times more likely to get pneumonia than children without rickets.
Effects of VDR-ligands, such as vitamin D hormone, on T-cells include suppression of T cell activation and induction of regulatory T cells, as well as effects on cytokine secretion patterns. VDR-ligands have also been shown to affect maturation, differentiation, and migration of dendritic cells, and inhibits DC-dependent T cell activation, resulting in an overall state of immunosuppression.
These immunoregulatory properties indicate that ligands with the potential to activate the VDR, including supplementation with calcitriol (as well as a number of synthetic modulators), may have therapeutic clinical applications in the treatment of inflammatory diseases (rheumatoid arthritis, psoriatic arthritis), dermatological conditions (psoriasis, actinic keratosis), osteoporosis, cancers (prostate, colon, breast, myelodysplasia, leukemia, head and neck squamous cell carcinoma, and basal cell carcinoma), and autoimmune diseases (systemic lupus erythematosus, type I diabetes); central nervous systems diseases (multiple sclerosis); and in preventing organ transplant rejection.
A 2006 study published in the Journal of the American Medical Association, reported evidence of a link between Vitamin D deficiency and the onset of multiple sclerosis; the authors posit that this is due to the immune-response suppression properties of Vitamin D. Further research indicates that vitamin D is required to activate a histocompatibility gene (HLA-DRB1*1501) necessary for differentiating between self and foreign proteins in a subgroup of individuals genetically predisposed to MS.
about Vitamin D and disease