Summary: Vitamin D ‒ a pleiotropic hormone, mainly synthetized in the skin following UVB exposure, then activated biologically undergoing two hydroxylations plays an important role in the regulation of calcium and phosphate metabolism, and in functioning of many other systems. A number of observational studies and clinical trials have been conducted to assess its impact on the immune function. The majority of immune cells express vitamin D receptor (VDR) and produce 1-α-hydroxylase, modulating both innate and adaptive immunity. The link between vitamin D deficiency and increased risk of infections, including COVID-19, is not unexpected, and has been observed in many studies. However, the trials aiming to determine the influence of vitamin D administration on the immune system have not given the clear answer. Deficiency of vitamin D is also observed to be associated with autoimmune diseases. In the article, the most important mechanisms by which 1,25(OH)2D may modulate the immune system and their clinical consequences are reviewed.