CD4 and Parkinson disease: DA receptors are expressed by both cluster of differentiation 4 (CD4)+ and 8 (CD8)+ T-cells (Baird et al., 2019; Liu et al., 2021) and CD4+ and CD8+ T-cells have been shown to infiltrate the midbrain in animal models of PD (Galiano-Landeira et al., 2020).