Vitamin D plays an ever clearer role in regulating physiological processes concerning systems and organs which are involved in calcium homeostasis, such as bones, intestines and kidneys. Indeed the vitamin D receptor (VDR) – the receptor at the level of the vitamin nucleus which mediates many, if not all, of the functions of its preferred ligand (1,25-dihydroxyvitamin D [1,25(OH)2D], or calcitriol) – is present in many body tissues. Many of these tissues contain the enzyme CYP27B1, which converts the most important circulating metabolite of vitamin D, 25-hydroxyvitamin D (25OHD), or calcifediol, into the active form 1,25(OH)2D. Vitamin D is able to influence the susceptibility and severity of infections through multiple mechanisms which involve the immune system in both its innate and adaptive forms. As a pleiotropic agent, vitamin D is able to activate memory T cells (Tregs), to modulate the action of the toll-like receptors (TLRs) present on the dendritic cells, to regulate the production of cytokines (decrease of inflammatory cytokines, increase of IL-10), and to activate factors of innate immunity such as cathelicidins and defensins.
