Taken together, the detection of CD4+ and CD8+ T cells within the SN of mice treated with MPTP and in PD patients, the proximity of infiltrating T cells to MHC expressing microglia/macrophages, and CD4/CD8 ratios of infiltrating T cells that are reversed from those expected in peripheral circulation[13, 51, 97, 104, 108] provide strong evidence for the directed extravasation and migration of activated T cells to sites of inflammation and for the association of increased disease or lesion progression with increased T cell infiltration. Here, CD4 is linked to Parkinson disease.