Females with severe COPD have more airway-associated lymphoid follicles in airway perimeter of 2-4 mm than those of males. Representative lung sections from a COPD patient stained with a) H&E and b) antibody against CD79α (B-cell marker; indicated by yellow triangle) in airway-associated lymphoid follicles are shown. c Percent airways with follicles are shown in lung tissue sections from subjects with GOLD 3–4 COPD in airways with perimeter 2-4 mm measured at the basement membrane (Pbm). Boxplots indicate the median and the interquartile range. Wilcoxon rank sum test was used in panel c. Human tonsil sections stained with d) secondary only (neg-negative control) and e) antibody against CD21 (follicular dendritic cell marker) were used as positive control. f Representative lung section from a COPD patient containing a lymphoid follicle stained with antibody against CD21 is shown. Representative lung sections from a g) control non-smoker and h) COPD patient stained with antibody against BAFF are shown.