IL21 and periodontitis: In periodontal lesions, Th17 cell-related cytokines (IL-17A, IL-6, IL-23, and IL-21) and osteoclastogenic mediators (RANKL) are significantly increased.44 Treg cells are an important subpopulation of immunosuppressive CD4 + T cells; they secrete anti-inflammatory cytokines (IL-10, IL-4, TGF-β) and downregulate RANKL expression, inhibiting alveolar bone resorption in periodontitis.45 Th17 cells and Treg cells have opposite immune regulation functions on bone metabolism.