CD4 and Parkinson disease: Inflammatory cells (e.g., microglia, CD4+ and CD8+ T cells; Marogianni et al., 2020) and inflammatory factors (e.g., IL-1α, IL-2, IL-1β, TNF-α, IL-6, TGF-β, IFN-γ, and IL-9; Karpenko et al., 2018) serve as key mediators between the brain and the immune system and play a pivotal role in the pathogenesis of PD.