B cells and T cells are primary RANKL expression cells in periodontal bone resorption. Th17 cells can express RANKL; however, RANKL expressed on Th17 cells alone is not sufficient to induce osteoclastogenesis. Treg cells may play a role in the downregulation of RANKL expression by activated lymphocytes in periodontal disease tissues, but the mechanism is unclear. Osteoblast stromal cells may also express RANKL, but they play a role in preventing bone resorption, since additional stimuli are required for the formation of active osteoclasts. Although macrophage may not be the main source of RANKL expression in periodontal disease, it can influence RANKL expression through its cytokines.