Thus, obesity-related differences in the induction of IL-13 after O3 exposure not only confer unique and/or augmented responses to O3 on the obese mice, but they also appear to account for some of the obesity-related differences in the impact of TNFR2 deficiency in obese mice because TNFR2 can synergize with IL-13 in obese but not lean mice. This evidence concerns the gene TNFRSF1B and obesity due to melanocortin 4 receptor deficiency.