Increasing evidence suggests that significantly increased T cells (CD8 and CD4) are present in the substantia nigra of PD patients compared with control subjects (Theodore et al., 2008; Harms et al., 2017), and T-cell responses were connected with dopaminergic neuron cell loss (Brochard et al., 2008; Lira et al., 2011; Williams et al., 2021). Here, CD4 is linked to Parkinson disease.