Sputum IL-33 levels were significantly increased in COPD subjects (6.52 ± 2.37 pg/ml) compared with healthy non-smokers (0.0 ± 0.0 pg/ml; p < 0.05) and healthy smokers (2.92 ± 2.45 pg/ml; p < 0.05) (Fig. 2b). Here, IL33 is linked to chronic obstructive pulmonary disease.