CD4 and Parkinson disease: In particular, in patients with PD, circulating CD4 and CD8 T cells infiltrate sites affected by the presence of α-syn and, in response, produce IFNγ and Th1/Th2 cytokines—an event in which the upward expression of MHCII influences myeloid cells of the CNS, thereby, allowing for the recognition of α-syn as a trigger of the immune response [96].