IL18 and chronic obstructive pulmonary disease: The area of IL-18+ immunoreactivity within each individual lymphoid aggregate was similar among the study groups (Fig. 3d; Kruskall Wallis test p = 0.19 not significant), however the total number of IL-18+ cells in the lung tissue overall was found to be increased in subjects with GOLD stage 4 COPD compared to controls because the total number of lymphoid structures in the lung tissue was inversely correlated to FEV1 as assessed by Spearman rank order correlation (Fig. 3e; rs = −0.464, p = 0.0012).