Figure 1.

Based on recent studies, we highlight a possible explanatory scheme for the relationship between hypovitaminosis D, dysbiosis, and alterations of the immune system associated with immune-mediated diseases. In particular, a reduction in the level of vitamin D could lead to a reduced immunological tolerance of adaptive immunity and a reduced activity of the innate immune system. The latter aspect may, in turn, lead to less effective control of the microbiome and possible bacterial translocation. Relating instead to the microbiome alone, the most common alterations associated with immune system diseases are described in the image. The arrows stand for the level of activity, or in quantitative terms, relative to the different aspects under consideration. Specifically, if reduced, the arrow is downward, and if increased, it is upward. Abbreviations: ILCs: innate lymphoid cells; Th: T helper; Treg: regulatory T cells; SCFAs: short-chain fatty acids. Created with BioRender.com.