IFNG and Parkinson disease: Significant correlation evident between serum IFNγ and TNFα levels in both healthy controls (Spearman r = − 0.569; P < 0.001) and individuals with other neurological disorders (r = 0.469; P < 0.001) was absent in PD patients (< 1 year PD, r = 0.212, P > 0.05; 1–3 years PD, r = 0.223, P > 0.05; 3–12 years PD, r = 0.068, P > 0.05).