In COPD GOLD II, there were positive correlations between the relative number of studied cells and serum levels of IL-21 (r = 0.85 and p < 0.05), IL-6 (r = 0.69 and p < 0.05), IL-17A (r = 0.67 and p < 0.05), and IL-10 (r = 0.53 and p < 0.05). Here, IL17A is linked to chronic obstructive pulmonary disease.