Vitamin D receptors have been identified in most immune cells, of which some can also synthesize the active form of vitamin D from its precursor, therefore suggesting that might have important immunoregulatory properties [99]. Vitamin D is synthesized at skin level in the presence of UV-light from cholesterol and it is also taken up from the diet (fish, eggs, fortified milk, and mushrooms). The active form of vitamin D, calcitriol (1,25-dihydroxy vitamin D3), formed following kidney and liver hydroxylation, is most renowned for its regulating role in calcium homeostasis and bone health status, but it has also been shown to regulate the immune system, mainly in the functioning of T-cell [102].