CRP and periodontitis: Furthermore, there are studies that have shown a significant increase of IL-23 levels in serum of patients with OSA, and its positive correlation with AHI and C-reactive protein (CRP) [31,32], suggesting that high levels of IL-23 in patients with OSA can stimulate the production of pro-inflammatory cytokines by TH17 cells, like IL-17A, which is involved in the development of periodontitis [29].