IL21 and periodontitis: Indirectly, these cells can also induce the amplification of the pro-inflammatory cytokines (IL-1, IL-6, IL-12, IL-17, IL-18, IL-21, TNF-α, and IFN-γ), which are known to be found in periodontitis [45] and to induce osteoclastogenesis via the RANKL–RANK–OPG pathway.