CD4 and Parkinson disease: Patients with Parkinson’s disease (PD) demonstrate a Th1 biased immune signature brought about by reduced Th2, Th17, and Tregulatory cells (Treg), leading to a relative increase in Th1 cells as well as the preferential differentiation of naïve CD4+ cells towards the Th1 lineage from the in vitro functional testing of PD patient cells [114].