For the purpose of this review, we focused on the effects of vitamin D in modulating the immune system. Several mechanisms have been described [83]: firstly, vitamin D was found to induce the production of antimicrobial peptides such as cathelicidin and human beta-defensin from immune system cells such as neutrophils and macrophages and from epithelial respiratory cells [81,82,83,84,85,86,87]. Vitamin D also enhances the antimicrobial activity of macrophages by increasing TLR and CD14 expression [88], autophagy [89,90], and the activity of NADPH-dependent oxidase [91]; it also promotes the migration of dendritic cells to lymphoid organs where they can present antigens to T cells [92]. On the other side of the coin, vitamin D can also inhibit the production of pro inflammatory cytokines, which might appear counterproductive [93]; it is known, however, that the pathogenicity of respiratory viruses, including SARS-CoV2, can be linked to hypercytokinemia up to the so-called “cytokine storm” [94,95,96,97,98]. This immunoregulatory effect of vitamin D can thus be beneficial to the host while facing a viral infection.