The vermiform appendix was once considered to be a vestigial (left over, no longer used) organ. There is now evidence that the appendix plays a major role in the regulation of the immune system especially regulation of gut bacteria.
From the largest of these epidemiologic related studies that reviewed the health records of 1.6 million Swedes that the incidence of PD was 19.3% lower in those that had their appendix removed.
This data suggests that the appendix may be a reservoir of alpha-synuclein (the protein that accumulates in excess in the brain's of Parkinson's and related conditions (DLB).
Since only 1% of the population gets PD it has been postulated that some, as yet unknown, confluence of events (such as an environmental trigger) that alters the gut microbiome decades before the disease is evident that mediates the risk of Parkinson's.
Sci Transl Med. 2018;10(465)