Inhibitor of differentiation 3 (Id3) is necessary for interleukin (IL)-33–induced IL-5 levels. A, IL-33 or PBS vehicle control was administered intraperitoneally to Id3+/+Apoe−/− (PBS group, n=7; IL-33, n=5) or Id3−/−Apoe−/− (PBS group, n=7; IL-33, n=6) mice every 2 days 3× in 4 independent experiments. On day 7, serum and peritoneal fluid were harvested and IL-5 levels were determined by ELISA. Values were considered not detectable (n.d.) if below the sensitivity of the assay (≤1.56 pg/mL). B, Id3+/+Apoe−/− or Id3−/−Apoe−/− mice were administered PBS vehicle control and IL-5 for 5 days, including BrdU on day 5. *P<0.05, **P<0.001, and ****P<0.0001.