There is further controversy on the proportions of CD4+ T cell subsets in the circulation of PD patients, as one study highlights an increase in Tregs (Bas et al., 2001) and others show a decrease and loss in functionality (Baba et al., 2005; Saunders et al., 2012; Chen et al., 2015; Sommer et al., 2019). This evidence concerns the gene CD4 and Parkinson disease.