CD38 and Parkinson disease: As shown in Fig. 4, we found a decrease in the number of plasma cells (CD19−CD138+CD38+) in the PD-2 yr group with respect to PD-1 yr (P = 0.048); interestingly, we found a positive correlation with age in the in PD-0 yr group (r = 0.461, P = 0.012) (Additional File 5: Table S4).