IL-4 produced by basophils is primarily responsible for basophil-induced Th2 differentiation. (A) Coculture experiments were performed in a Transwell experiment. Naive 5C.C7 CD4 T cells were stimulated with 2 × 10⁴ basophils together or separated from basophils by Transwell. Production of IL-4 and IFNγ was determined. (B) Purified OT-II CD4 T cells specific for OVA peptide in the context of I-A^b were cultured with OVA peptide 323 to 339–loaded splenic CD11c⁺ dendritic cells in the absence or presence of basophils isolated from wild-type mice or IL-4–deficient G4/G4 mice for 72 hours. Cytokine production was measured. (C) During primary stimulation, 10 μg/mL anti–IL-3 or anti–IL-13 antibodies were added. All the experiments were repeated more than 3 times with similar results.