Studies have shown that peripheral antigen-presenting cells (APCs) could migrate across blood-brain barrier (BBB) and intake α-synuclein accumulates in the substantia nucleus (SN) of PD [20], and then present them to CD4+ and CD8+ T cells, so α-synuclein serve as a major antigen to active antigen-specific T cells [20]. Here, CD4 is linked to Parkinson disease.