Prophylactic prevention of infection requires dietary vitamin C intakes that provide at least adequate, if not saturating plasma levels (i.e., 100–200 mg/day), which optimize cell and tissue levels. Furthermore, supplementation with vitamin C appears to be able to both prevent and treat respiratory and systemic infections. In turn, infections significantly impact on vitamin C levels due to enhanced inflammation and metabolic requirements. Vitamin C deficiency results in impaired immunity and higher susceptibility to infections. The role of vitamin C in lymphocytes is less clear, but it has been shown to enhance differentiation and proliferation of B- and T-cells, likely due to its gene regulating effects. It is also needed for apoptosis and clearance of the spent neutrophils from sites of infection by macrophages, thereby decreasing necrosis/NETosis and potential tissue damage. Vitamin C accumulates in phagocytic cells, such as neutrophils, and can enhance chemotaxis, phagocytosis, generation of reactive oxygen species, and ultimately microbial killing. Vitamin C supports epithelial barrier function against pathogens and promotes the oxidant scavenging activity of the skin, thereby potentially protecting against environmental oxidative stress.

Vitamin C contributes to immune defense by supporting various cellular functions of both the innate and adaptive immune system.

It is a potent antioxidant and a cofactor for a family of biosynthetic and gene regulatory enzymes. Vitamin C is an essential micronutrient for humans, with pleiotropic functions related to its ability to donate electrons.

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