Between Deficiency and Excess: The Dual Role of Selected Dietary Supplements in Immune Health

The immune system is a complex, tightly regulated network maintaining host defense, self-tolerance, and physiological homeostasis. Its effectiveness depends on the coordination of innate and adaptive responses, both significantly influenced by nutritional status. Recently, dietary supplements have gained attention as modulators of immune function, particularly in populations at risk of micronutrient deficiencies. This narrative review evaluates current evidence on the role of zinc, selenium, and vitamin C in immune health. These micronutrients were selected for their established roles in immune cell development, antioxidant defense, and inflammatory regulation. The review analyzes experimental, clinical, and epidemiological studies to assess their mechanisms, clinical efficacy, and safety. Zinc is essential for both innate and adaptive immunity, playing a pivotal role in T-lymphocyte maturation, macrophage and neutrophil function, and cytokine production. While zinc deficiency increases infection susceptibility, supplementation benefits primarily those with deficient or high-risk status. Selenium, a key component of selenoproteins like glutathione peroxidase, supports antioxidant defense and modulates cell-mediated immunity. Evidence indicates that selenium may enhance immune function in deficient individuals; however, excessive intake carries a narrow therapeutic window and potential toxicity. Vitamin C contributes to defense by maintaining epithelial barrier integrity, supporting leukocyte chemotaxis, and modulating oxidative stress. While routine high-dose supplementation offers limited benefits in healthy individuals, adequate intake remains crucial for those with low dietary consumption. This review highlights the risks associated with indiscriminate supplementation. While zinc, selenium, and vitamin C are indispensable for optimal immunity, their use should be individualized and targeted toward correcting confirmed deficiencies. A personalized, evidence-based approach remains essential for supporting immune health while minimizing potential risks.