CD4 and Parkinson disease: To investigate how clinical parameters of PD relate to the changes observed in the numbers of T cells in PD, we investigated the relative abundance of total CD3+ T cells, as well as the subpopulations of CD3+CD4+ and CD3+CD8+ T cells in a cross-sectional cohort of 36 PD patients (mean age 70.3 ± 9.5; f/m = 12/24; mean H&Y 2.4 ± 0.94) and 20 age-matched (p > 0.32) healthy controls (mean age 67.8 ± 8.0; f/m = 13/7) (Fig. 1A).