CCL22 and Parkinson disease: Linear regressions assessing the relationship between CCL22 content and ferritin abundance separated by disease group revealed a substantial relationship in IBD (R2 = 0.60, p= 0.0020) and PD (R2 = 0.51, p= 0.006) plasma samples, where individuals with low CCL22 also had low ferritin but this relationship did not exist in NHC (R2 = 0.19, p= 0.21) (Fig. 5Aii).