Vitamin D belongs to the group of fat–soluble secosteroids. Found in 1782 and isolated not until the early 1920s, it was chemically characterized in 1931. In recent years, more emphasis has been given on the three mechanisms of action of vitamin D: endocrine (regulating the calcium absorption), autocrine/paracrine (facilitating the gene expression) and immune (activation of macrocytes and other cells) (1–5).