CXCL8 and Parkinson disease: The results of studies on the serum levels of cytokines such as IFN-γ and TNF-α secreted by Th1 T cells, IL-8 and IL-10 secreted by Th2 T cells, and IL-17 secreted by Th17 in PD patients are not uniform [19,22,23,24,25,26,27].