IL17A and asthma: However, our results are inconsistent with the discoveries of Castro et al’s (42) that treatment with vitamin D had no significant effect on lung function or airway hyperreactivity, and neither asthma quality of life nor asthma control was improved with vitamin D. IL-17A has been established as an independent risk factor for severe asthma, and has been associated with a T2-low endotype and neutrophilic phenotype (43, 44).