Vitamin C is an essential micronutrient with diverse functions, including antioxidant activity and roles in biosynthetic and gene regulatory enzymes. It supports immune defense by enhancing innate and adaptive immune functions, epithelial barrier integrity, and microbial killing. Vitamin C is crucial for collagen synthesis, wound healing, and maintaining skin barrier function. It also plays a role in neutrophil function, including chemotaxis, phagocytosis, and apoptosis, which are essential for immune responses. Vitamin C deficiency impairs immunity and increases susceptibility to infections, while infections can further deplete vitamin C levels due to increased inflammation and metabolic demands. Supplementation with vitamin C can prevent and treat respiratory and systemic infections, with prophylactic doses of 100–200 mg/day optimizing immune function. Treatment of established infections requires higher doses to counteract increased inflammatory responses. Vitamin C also modulates lymphocyte function, enhancing B- and T-cell differentiation and proliferation. It influences gene expression and epigenetic regulation, affecting immune responses. Vitamin C deficiency is common in Western populations and is linked to increased infection risk. Environmental factors, such as air pollution and tobacco smoke, can exacerbate vitamin C insufficiency. Conditions like diabetes, aging, and chronic diseases are associated with lower vitamin C levels and increased infection risk. Vitamin C supplementation has shown benefits in reducing infection severity and duration, particularly in respiratory infections. Overall, vitamin C is essential for immune function, helping the body defend against pathogens while minimizing tissue damage.Vitamin C is an essential micronutrient with diverse functions, including antioxidant activity and roles in biosynthetic and gene regulatory enzymes. It supports immune defense by enhancing innate and adaptive immune functions, epithelial barrier integrity, and microbial killing. Vitamin C is crucial for collagen synthesis, wound healing, and maintaining skin barrier function. It also plays a role in neutrophil function, including chemotaxis, phagocytosis, and apoptosis, which are essential for immune responses. Vitamin C deficiency impairs immunity and increases susceptibility to infections, while infections can further deplete vitamin C levels due to increased inflammation and metabolic demands. Supplementation with vitamin C can prevent and treat respiratory and systemic infections, with prophylactic doses of 100–200 mg/day optimizing immune function. Treatment of established infections requires higher doses to counteract increased inflammatory responses. Vitamin C also modulates lymphocyte function, enhancing B- and T-cell differentiation and proliferation. It influences gene expression and epigenetic regulation, affecting immune responses. Vitamin C deficiency is common in Western populations and is linked to increased infection risk. Environmental factors, such as air pollution and tobacco smoke, can exacerbate vitamin C insufficiency. Conditions like diabetes, aging, and chronic diseases are associated with lower vitamin C levels and increased infection risk. Vitamin C supplementation has shown benefits in reducing infection severity and duration, particularly in respiratory infections. Overall, vitamin C is essential for immune function, helping the body defend against pathogens while minimizing tissue damage.