CD4 and Parkinson disease: According to the whole picture, CD4+ T cells reduction in PD patients is accounted for by reduced Th2 (on average, − 23.7 × 106/L), Th17 (− 31.1 × 106/L), Th1/17 (− 33.8 × 106/L), Treg (− 30.4 × 106/L), and T naïve cells (− 117.2 × 106/L), which altogether make a total of 236.2 × 106/L less cells, an amount well matching with the global differences above reported for total CD4+ T cells.