 - 3 - Vitamin D and autoimmunity Vitamin D, a steroid hormone, is well known to be involved in calcium and phosphate homeostasis and bone metabolism 14.The target organs for the non-classical actions of the vitamin D include immune systems, pancreatic b-cells, the heart and cardiovascular system, the brain and reproductive tissues. Tissue responses to vitamin D include regulation of hormone secretion modulation of immune responses, and a control of cellular proliferation and differentiation 15. Vitamin D was also reported to inhibit proliferation of T helper 1 Th1 cells and limit their production of cytokines, such as interferon gamma IFN-g, interleukin-2 IL-2 and tumor necrosis factor-alpha TNF-a.Conversely, vitamin D induces T helper 2 Th2 cytokines, such as IL-4, IL-5, IL-6, IL-9, IL-10 and IL-13 16. Furthermore, in many studies vitamin D has been presented as a modifiable environmental factor for Th1-mediated autoimmune disease and appears to be important for susceptibility to and severity of the disease 17. Vitamin D also regulates B cell immunity. It down-regulates the proliferation and differentiation of B lymphocytes and inhibits IgG production 16. 