Vitamin D and the immune system: beyond rickets.

In addition to its essential role in calcium homeostasis and bone metabolism, vitamin D has a diverse range of biological actions, including induction of cell differentiation, inhibition of cell growth, immunomodulation and control of hormonal systems. Vitamin D plays an immunoregulatory role in both the innate and adaptive immune systems. The active metabolite of the vitamin D endocrine system, 1,25-dihydroxyvitamin D (calcitriol), exerts pleiotropic effects through its interaction with the vitamin D receptor. Low vitamin D status in humans has been implicated in the etiology of neoplasia, autoimmune disease, cardiovascular disorders and infectious diseases. This review focuses on vitamin D and its effects on immune function, particularly in humans, with the aim to encourage further exploration in the veterinary field.