Green Tea (Camellia sinensis) Consumption and Human Immune Function: A Systematic Review of Immunomodulatory and Anti-Inflammatory Effects

Background: Green tea (Camellia sinensis) is widely consumed around the world, particularly in East Asia, and has traditionally been used to support cardiovascular and metabolic health. Rich in catechins—most notably epigallocatechin-3-gallate (EGCG)—green tea is recognized for its potential immunomodulatory and anti-inflammatory effects. However, evidence from human studies remains inconsistent, and the distinction between functional immune outcomes and systemic inflammatory markers has not been systematically evaluated. Methods: We conducted a systematic review in accordance with PRISMA guidelines, searching PubMed, Google Scholar, Cochrane Library, and EBSCO from inception to the most recent date. Eligible studies included human investigations evaluating green tea or catechin intake and reporting either immune response outcomes (e.g., natural killer cell activity, T cell responses) or systemic inflammatory markers (e.g., C-reactive protein, cytokines). Randomized controlled trials (RCTs), non-randomized interventional studies, and observational analyses were included. Data were synthesized narratively due to heterogeneity. Results: Nineteen studies (≈6800 participants) were included, comprising 12 RCTs, three non-randomized interventions, and five observational studies. Eight studies assessed immune responses and consistently demonstrated enhanced immune activity, including increased natural killer cell function and augmented γδ T cell responses. Eleven studies evaluated inflammatory markers and showed heterogeneous results: several trials reported reductions in C-reactive protein and related biomarkers, particularly in high-risk populations, whereas others found no significant changes in cytokines such as interleukin-6 or tumor necrosis factor-α. Observational studies generally reported inverse associations between tea consumption and inflammation. Green tea was well tolerated across studies. Conclusions: Green tea consumption appears to enhance functional immune responses while exerting modest, context-dependent anti-inflammatory effects. These findings support a dual immunomodulatory role and highlight the importance of distinguishing between immune function and systemic inflammation in nutritional research.