The Role of Vitamin D in Immune System Regulation and Respiratory Health: A Clinical Review

Background: Hypovitaminosis D is a global public health pandemic affecting over one billion individuals. Classically recognized for musculoskeletal integrity, contemporary clinical research demonstrates that vitamin D functions as a pleiotropic secosteroid hormone with profound immunomodulatory properties.Objective: This clinical review synthesizes current molecular, immunological, and clinical evidence elucidating the role of vitamin D in innate and adaptive immune regulation, with particular focus on host defense mechanisms against acute respiratory tract infections (ARTIs).Findings: Calcitriol induces the gene expression of antimicrobial peptides, notably cathelicidin (LL-37) and human beta-defensin 2, while suppressing excessive Th1/Th17 cascades and promoting regulatory T cells (Tregs). Meta-analyses demonstrate that vitamin D supplementation significantly reduces the incidence and severity of acute respiratory tract infections, especially in deficient individuals.Conclusion: Vitamin D is an essential, cost-effective immunomodulatory adjuvant in respiratory medicine.