CD8+ and CD4+ T lymphocytes enter the brain in the 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) PD animal model and postmortem PD specimens (48), and lower plasma lymphocyte counts in patients with PD may be linked to BBB malfunction, which results in lymphocyte recruitment to the brain parenchyma. This evidence concerns the gene CD4 and Parkinson disease.