The current study found that several immune cells (e.g., central memory CD4 T cells, effector memory CD8 T cells, activated CD8 T cells, myeloid-derived suppressor cells (MDSCs), plasmacytoid dendritic cells, activated dendritic cells, and Monocytes) were highly expressed in periodontitis disease and PD. The gene discussed is CD8A; the disease is Parkinson disease.