Vitamin D plays a crucial role in immune function, influencing both innate and adaptive immunity. It is present in various immune cells, including antigen-presenting cells, T cells, B cells, and monocytes. In vitro studies show that vitamin D modulates innate immune cells and promotes a more tolerogenic immune status. In vivo studies in animals and humans demonstrate beneficial effects of vitamin D on immune function, particularly in autoimmune diseases such as type 1 diabetes and multiple sclerosis. Vitamin D is synthesized in the skin upon exposure to UVB radiation and can also be obtained from dietary sources and supplements. The main circulating form of vitamin D is 25-hydroxyvitamin D (25(OH)D), which is converted in the kidneys to the active form, calcitriol. Vitamin D metabolism involves enzymes such as CYP27B1 and CYP24A1, which regulate the conversion and inactivation of vitamin D. Vitamin D status is defined by serum 25(OH)D levels, with thresholds varying between different organizations. The Endocrine Society recommends levels below 20 ng/mL as deficient, while the Institute of Medicine considers levels above 20 ng/mL as sufficient. Vitamin D supplementation is generally safe, with recommended upper limits varying between 4000 and 10,000 IU/day. Vitamin D has immunomodulatory effects on the innate immune system by enhancing antimicrobial responses and promoting a tolerogenic immune status. It influences the function of immune cells such as macrophages and dendritic cells, modulating their activity and promoting the production of antimicrobial peptides. Vitamin D also affects the adaptive immune system by regulating T cell function, promoting the production of anti-inflammatory cytokines, and inhibiting pro-inflammatory cytokines. In autoimmune diseases, vitamin D has been shown to reduce inflammation and promote immune tolerance. Studies suggest that vitamin D deficiency is associated with an increased risk of autoimmune diseases such as type 1 diabetes and multiple sclerosis. Clinical trials have shown that vitamin D supplementation may reduce the risk of autoimmune diseases and improve immune function. Vitamin D is a promising and relatively safe nutrient for the prevention and adjunctive treatment of diseases caused by impaired immune homeostasis. Further research is needed to determine the optimal mode and dosage of vitamin D supplementation for different diseases.Vitamin D plays a crucial role in immune function, influencing both innate and adaptive immunity. It is present in various immune cells, including antigen-presenting cells, T cells, B cells, and monocytes. In vitro studies show that vitamin D modulates innate immune cells and promotes a more tolerogenic immune status. In vivo studies in animals and humans demonstrate beneficial effects of vitamin D on immune function, particularly in autoimmune diseases such as type 1 diabetes and multiple sclerosis. Vitamin D is synthesized in the skin upon exposure to UVB radiation and can also be obtained from dietary sources and supplements. The main circulating form of vitamin D is 25-hydroxyvitamin D (25(OH)D), which is converted in the kidneys to the active form, calcitriol. Vitamin D metabolism involves enzymes such as CYP27B1 and CYP24A1, which regulate the conversion and inactivation of vitamin D. Vitamin D status is defined by serum 25(OH)D levels, with thresholds varying between different organizations. The Endocrine Society recommends levels below 20 ng/mL as deficient, while the Institute of Medicine considers levels above 20 ng/mL as sufficient. Vitamin D supplementation is generally safe, with recommended upper limits varying between 4000 and 10,000 IU/day. Vitamin D has immunomodulatory effects on the innate immune system by enhancing antimicrobial responses and promoting a tolerogenic immune status. It influences the function of immune cells such as macrophages and dendritic cells, modulating their activity and promoting the production of antimicrobial peptides. Vitamin D also affects the adaptive immune system by regulating T cell function, promoting the production of anti-inflammatory cytokines, and inhibiting pro-inflammatory cytokines. In autoimmune diseases, vitamin D has been shown to reduce inflammation and promote immune tolerance. Studies suggest that vitamin D deficiency is associated with an increased risk of autoimmune diseases such as type 1 diabetes and multiple sclerosis. Clinical trials have shown that vitamin D supplementation may reduce the risk of autoimmune diseases and improve immune function. Vitamin D is a promising and relatively safe nutrient for the prevention and adjunctive treatment of diseases caused by impaired immune homeostasis. Further research is needed to determine the optimal mode and dosage of vitamin D supplementation for different diseases.