IL33 and obesity due to melanocortin 4 receptor deficiency: In contrast, serum IL-33 was unchanged by obesity (7.1 ± 0.8 versus 6.0 ± 0.7 pg/mL in O3-exposed db/db versus WT mice, respectively) and was approximately 50% lower after O3 than air in both WT and db/db mice (data not shown).