CD4 and Parkinson disease: A recent study showed that T cells from patients with PD recognize α-synuclein (Sulzer et al., 2017), and non-classical monocytes and reactive CD4+ and CD8+ reactive T cells are found in post-mortem brain tissue (Brochard et al., 2009; Sommer et al., 2018), indicating elevated inflammation as well as adaptive immune cell invasion into PD-patient brains.