**Abstract**

Vitamin D has many and profound effects on the immune system. Vitamin D deficiency is known to be related to the development of autoimmune diseases. In particular, vitamin D deficiency is related to the development and the severity of rheumatoid arthritis (RA). RA develops in patients with vitamin D deficiency, and the activity of the disease is related to vitamin D deficiency. Vitamin D deficiency is also related to the development of systemic lupus erythematosus (SLE). SLE develops in patients with vitamin D deficiency, and the activity of the disease is also greater in patients with vitamin D deficiency. Vitamin D deficiency is also related to the development and the severity of multiple sclerosis. Vitamin D should be administered to patients with multiple sclerosis, and this seems to mitigate the symptoms of the disease and to prevent disease progression. Vitamin D deficiency is also observed in patients with inflammatory bowel disease and may be related to disease severity. Low vitamin D levels have also been observed in patients with autoimmune Hashimoto's thyroiditis. Low vitamin D levels have been observed in patients with systemic sclerosis, especially in the diffuse form of the disease. Optimal vitamin D levels appear to be required for normal immune function and for the prevention and treatment of autoimmune diseases.

**Keywords:** vitamin D, autoimmunity, rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, inflammatory bowel disease, autoimmune Hashimoto's thyroiditis
