vitamin D has been at the center of a resurgence of research activity during the last two decades, with particular attention to the role it plays in development and function of the immune system. Vitamin D (used to describe either vitamin D2 or D3 unless otherwise specified) enters the body in two ways: exposure to the sun or consumption of fortified foods and feeds, oily fish such as salmon, or yeasts and mushrooms exposed to ultra-violet light B (UVB light). The primary endogenous source of vitamin D is production by the epidermis exposed to UVB sunlight, which produces the vitamin D precursor known as cholecalciferol. Cholecalciferol is transported to the liver where it is converted to 25-hydroxy vitamin D (25-OHD).1 Commonly regarded as the storage form of vitamin D, 25-OHD is most useful in determining vitamin D status of the animal.2 It is converted into the active form, 1,25(OH)2D by the enzyme 1α-hydroxylase. Previously thought to be only in the kidney and bone where it is involved in mineral homeostasis, this enzyme is now known to exist in various tissues of the body, including those of the immune system.3