The findings of an immunophenotyping study suggest that abnormal peripheral B cell responses could contribute to Parkinson disease (PD). In a comparison of 61 people with PD and 61 healthy individuals, PD was associated with reduced B cell counts in the blood, but within the B cell population, the proportions of regulatory B cells, plasmablasts, double-negative B cells and B cells that produce various cytokines were increased. Increased blood levels of cytokines that regulate B cells, including APRIL, BAFF and sCD40L, were also associated with PD.