HBD and Parkinson disease: Based on the loading values of the proteins contributing to PC1, the top 10 (CELSR2, FGG, FGA, FGB, IGKV1, CA1, BLVRB, IGLV3, PRSS3, and HBD), high in PD, and bottom 10 (PPBP, PRTN3, TREML1, GP5, MPO, THBS1, PF4, APOF, AZU1, and PF4V1) proteins, low in PD, were displayed (Fig. 4f) and had significant quantitative differences from the rest (P < 0.05), except for one, PRSS3, which was significantly different from MS (P < 0.05) but not dementia (Supplementary Fig. 6).