Allergic disease as a risk factor for Parkinson’s disease: a possible role of eosinophil

Background Neuroinflammation involving microglial activation and α-synuclein accumulation is central to Parkinson’s disease (PD) pathogenesis. Chronic inflammation in allergic diseases such as asthma and allergic rhinitis may contribute to PD risk through shared immune pathways, but evidence is limited. Methods We conducted a prospective cohort study using the Korean National Health Insurance Service database. A total of 5,036,518 adults aged ≥40 years who underwent health examination in 2009 were followed for incident PD from 2010 to 2019, excluding prior PD. Allergic diseases (asthma, allergic rhinitis, atopic dermatitis) were defined by ICD-10 codes and healthcare utilization. Severity was assessed by outpatient visit, and multimorbidity by coexisting conditions. Cox proportional hazards models estimated hazard ratios (HRs) adjusting for demographics, lifestyle, and comorbidities. Additionally, we compared peripheral eosinophil and neutrophil counts between 234 hospital-based PD patients and 468 matched controls. Results Over 10.1 years, 44,621 PD cases occurred (0.9 per 1,000 person-years). PD risk was increased in asthma (HR 1.16; 95% CI 1.07–1.24) and allergic rhinitis (HR 1.18; 95% CI 1.14–1.22), but not atopic dermatitis. Risk rose with multiple allergic conditions (p trend