Previous studies found an ongoing loss of CD4+ T cells as the disease progresses in PD patients (Stevens et al., 2012), and PD patients with cognitive impairment had a higher number of circulating lymphocytes than patients with normal cognitive function (Magistrelli et al., 2020), and the heterogeneity of the PD patients may account for the discrepancies. Here, CD4 is linked to Parkinson disease.