OVA-incorporated BMMCs and peritoneal mast cells are a potent source of antigen for presentation to T cells. (A) Peritoneal mast cells from MHC class II^−/− mice were pretreated with (bottom) or without (top) anti-OVA IgE, washed, and incubated with OVA (50 μg/mL) for 2 days. The peritoneal mast cells were then washed and cocultured with OT-II splenocytes. (B) OT-II splenocytes were treated without (top left) or with OVA at 5 μg/mL (bottom left) or 0.5 μg/mL (top right). Anti-OVA IgE-pretreated MHC class II^−/− BMMCs were incubated with OVA (0.5 μg/mL) for 2 days, washed, and cocultured with the same OT-II splenocytes (bottom right). CD69 was detected 48 hours later on CD4⁺ T cells by flow cytometry. The number in the upper right corner indicates the percentage of CD69⁺CD4⁺ T cells of total CD4⁺ T cells.