BCR-Iga/gB -coreceptor complex B cell epitope (cD21cD19-cD81) coreceptor C3d B cell epitope BCR Ag-C3d CR2 BCR CR2 Iga Igβ CD19 ga/b CD81 CD19 Signal ITAM ignal 3P13K B cell activation ++十B cell activation B cell epitope BCR-Iga/Igb -coreceptor complex B cell epitope BCR Iga/b Signal + Ag-C3d CR2 CD19 Signal + +++ （CD21-CD19-CD81） coreceptor