Whilst I have covered the role of the lipid soluble vitamins A and D in various articles in terms of their immune modulating effects, a research paper out in the Nov 2012 Journal, Nature helps to expand some of the relationships between certain B vitamins and immune functionality.
Riboflavin (B2) and Folic Acid (B12) appear to act as a key precursor molecule for immune activation following their bacterial metabolism in the gut. Specialised mucosal immune cells, known as mucosa associated invariant T cells are then activated as the immune system uses the vitamin metabolites attached to the infected cells as a trigger.
In effect vitamins in this form act as antigens (agents that activate T and B cells) to promote immune defences through the activation of a group of unconventional T cells found mostly in the mucosal tissues of the gut, lings and liver. The role of MAIT cells in health and disease is unclear, although they can have crucial functions in protective immunity.
Intriguingly, and uniquely among known T-cell populations, MAIT-cell survival also depends on the commensal microbiota — the non-pathogenic microorganisms that live on and in the body. Furthermore, MAIT cells are activated by the presence of cells infected with a diverse range of bacteria and yeast strains (although not viruses).
Of note, these vitamin-based metabolites are secreted and are diffusible, suggesting a mechanism by which MAIT cells might sense bacterial activity across mucosal membranes potentially regulating local immunity and mucosal barrier functions.
Another pertinent question is that of the roles that MAIT cells have in the gut. Vitamins help to orchestrate the relationships between mammalian host immunity, commensal gut microbiota and pathogenic microorganisms. For example, vitamin B9 and its derivatives can serve as coenzymes in essential metabolic pathways, and this vitamin is also required for the survival of a type of T cell called regulatory T cells.
Although further work is required, it is attractive to speculate that MAIT-cell-dependent protection against pathogens could be augmented by dietary provision of vitamins. Such augmentation might enhance immunity to microbes, or even help to treat immunodeficiencies through the utilisation of safe oral supplementation.
 Kjer-Nielsen L, Patel O, Corbett AJ, Le Nours J, Meehan B, Liu L, Bhati M, Chen Z, Kostenko L, Reantragoon R, Williamson NA, Purcell AW, Dudek NL, McConville MJ, O’Hair RA, Khairallah GN, Godfrey DI, Fairlie DP, Rossjohn J, McCluskey J. MR1 presents microbial vitamin B metabolites to MAIT cells. Nature. 2012 Nov 29;491(7426):717-23. doi: 10.1038/nature11605. Epub 2012 Oct 1 View Abstract
 Treiner E, Duban L, Moura IC, Hansen T, Gilfillan S, Lantz O. Mucosal-associated invariant T (MAIT) cells: an evolutionarily conserved T cell subset. Microbes Infect. 2005 Mar;7(3):552-9. Epub 2005 Feb 12. Review View Abstract
 Gold, M. C. et al. Human mucosal associated invariant T cells detect bacterially infected cells. PLoS Biol. 8, e1000407 (2010) View Abstract
 Le Bourhis L, Martin E, Péguillet I, Guihot A, Froux N, Coré M, Lévy E, Dusseaux M, Meyssonnier V, Premel V, Ngo C, Riteau B, Duban L, Robert D, Huang S, Rottman M, Soudais C, Lantz O. Antimicrobial activity of mucosal-associated invariant T cells. Nat Immunol. 2010 Aug;11(8):701-8. Epub 2010 Jun 27. Erratum in: Nat Immunol. 2010 Oct;11(10):969. View Abstract
 Nicholson JK, Holmes E, Kinross J, Burcelin R, Gibson G, Jia W, Pettersson S. Host-gut microbiota metabolic interactions. Science. 2012 Jun 8;336(6086):1262-7. Epub 2012 Jun 6. Review. View Abstract