When patients were stratified according to disease severity, we found that the serum IL-31 levels for patients with severe asthma were higher than those in mild asthma (133.0 [103.0–180.1] pg/ml versus 86.8 [46.4–150.5] pg/ml; p = 0.011) and those in the controls (133.0 [103.0–180.1] pg/ml versus 52.5 [38.5–62.7] pg/ml; p < 0.001) (Fig. 1C). Here, IL31 is linked to asthma.