CD4 and Parkinson disease: Several studies demonstrated impaired frequencies of CD4+ T cells [12] and reduced CD4+: CD8+ T cell ratios due to decreased percentages of Th lymphocytes and increased percentages of Tc lymphocytes [13–15], as well as a decrease of CD4+ CD45RA+ “naive” T cells, and an increase of CD4+ CD45RO+ “memory” T cells in the blood of 64 [13] and 72 PD patients [12] (Table 1).