The Role of Vitamin D in Immune System Modulation: Mechanisms, Clinical Correlates, and Current Controversies

Vitamin D, a fat-soluble secosteroid, has long been associated with calcium homeostasis and bone metabolism. Over the past two decades, however, accumulating evidence suggests that its influence extends considerably into immune regulation, affecting both innate and adaptive immune responses. The vitamin D receptor (VDR), expressed on virtually all immune cell types, appears to mediate a broad spectrum of immunomodulatory effects, ranging from the transcription of antimicrobial peptides to modulation of T-regulatory cell populations. Epidemiological studies have consistently linked suboptimal vitamin D status with heightened susceptibility to respiratory infections, autoimmune conditions, and chronic inflammatory states, although establishing causality remains challenging. Randomized controlled trials have yielded heterogeneous results, and questions regarding optimal serum thresholds, supplementation dosing, and population-specific responses remain unresolved. This narrative review synthesizes current evidence on the immunological roles of vitamin D, with emphasis on mechanistic pathways, clinical correlates, and the methodological limitations that complicate interpretation of the existing literature.