Remarkably, Saunders et al. 12 recently reported that PD patients have increased effector/memory CD4+ T cells and decreased CD31+ and α4β7+ CD4+ T cells, which are associated with progressive motor dysfunction, suggesting a direct relationship between chronic immune stimulation and PD neuropathology and disease severity, as well as strengthening the idea that in PD the lead actors among adaptive immune system cells are CD4+ T lymphocytes. The gene discussed is PECAM1; the disease is Parkinson disease.