CD4 and Parkinson disease: Over the last 15 years, several studies described the occurrence of peculiar modifications of peripheral immunity in PD, such as decreased CD4+ /CD8+ T-cell ratios, fewer CD4+ CD25+ T cells and increased ratios of interferon (IFN)-γ-producing to interleukin (IL)-4-producing T cells8, as well as decreased CD4+ T lymphocytes and CD19+ B cells9, 10.