Conversely, impaired IL-10 production by TH cells has been reported in patients with severe asthma,22, 23 and genetic variants affecting IL-10 expression have been associated with asthma incidence and severity.24, 25 However, despite these associations, the mechanisms by which IL-10 regulates diverse immunologic phenotypes of asthma are incompletely understood.18 This evidence concerns the gene IL10 and asthma.