In the past, vitamin D was known for its classical, skeletal action as a regulator of calcium and bone homoeostasis. Currently, vitamin D was found to have a role in numerous physiological processes in the human body; thus, vitamin D haspleiotropic activity. The studies carried out in the past two decades showed the role of vitamin D in the regulation of immune system functions. Basically, these effects may be mediated not only via endocrine mechanism of circulating calcitriol but also via paracrine one (based on cell–cell communication that leads to production of signal inducing the changes in nearby/adjacent cells and modulating their differentiation or behaviour) and intracrine mechanism (the action of vitamin D inside a cell) of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) synthetized from its precursor 25-hydroxy vitamin D3 (25(OH)D3).Both vitamin D receptor (VDR) and 25-hydroxyvitamin D3 1-a-hydroxylase(CYP27B1) are expressed in several types of immune cells (i.e. antigen presenting cells, T and B cells), and thus, they are able to synthetize the bioactive form of vitamin D that modulates both the innate and adaptive immune system.This review discusses the role of vitamin D as regulator of immune system, and our understanding of how vitamin D regulates both adaptive and innate immunity as well as inflammatory cascade on the cellular level.
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