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

Neuroinflammation is central to Parkinson's disease (PD) pathogenesis and allergic inflammation may contribute to PD risk, but evidence is limited. Using the Korean National Health Insurance Service database, we conducted a cohort study of 5,036,518 adults aged ≥40 years who underwent health examination in 2009 and identified incident PD from 2010 to 2019, excluding prior PD. Asthma, allergic rhinitis, and atopic dermatitis were defined by ICD-10 codes and healthcare utilization, and severity by annual outpatient visits. Cox proportional hazards models estimated hazard ratios (HRs) adjusting for demographics, lifestyle, and comorbidities. We compared eosinophil and neutrophil counts between 234 hospital-based PD patients and 468 controls. During follow-up, 44,621 PD cases occurred (0.9/1000 person-years). PD incidence 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 increased with multiple allergic conditions (p_trend <0.001) and higher visit frequency (≥3/year: HR 1.26; 95% CI 1.16-1.38). In the hospital cohort, PD patients showed higher eosinophil and neutrophil counts, elevated eosinophil/neutrophil ratio, and more frequent eosinophilia ≥200 cells/µL (all p < 0.05). Asthma and allergic rhinitis were associated with increased PD incidence, particularly with greater severity and multimorbidity, supporting shared inflammatory mechanisms.