Fig. 1.

Risk and protective factors associated with immune system function and PD development. Autoimmune conditions, except for systemic lupus erythematosus, are identified as risk factors for the development of Parkinson’s disease (PD). Genes identified through GWAS are highlighted as associated with Crohn’s disease, rheumatoid arthritis, and ulcerative colitis, as well as with PD. LRRK2 is specifically highlighted given its strong association with PD as well as Crohn’s disease. Among infections, Hepatitis B and hepatitis C viral infection, as well as H. pylori show the most consistent evidence and are highlighted. Type 2 diabetes and irritable bowel syndrome, which include immune system dysfunction have been described as risk factor for PD. Finally, immunosuppressant medications are used to treat autoimmune conditions and, along with NSAIDs, have an inverse association with PD. Risk factors are shown in orange and protective factors in green. Lighter shaded boxes represent lower level of evidence (e.g., only one study describing the association) while heavier shading represents robust evidence (e.g., those where several epidemiological studies are available and consistent).