Previous findings suggest that during cognate T cell-B cell interactions, major histocompatability complex (MHC) class II molecules transduce signals, leading to Src-family kinase activation, Ca2+ mobilization, and proliferation. Here, we show that antigen stimulation of resting B cells induces MHC class II molecules to associate with Immunoglobulin (Ig)-α/Ig-β (CD79a/CD79b) heterodimers, which function as signal transducers upon MHC class II aggregation by the T cell receptor (TCR). The B cell receptor (BCR) and MHC class II/Ig-α/Ig-β are distinct complexes, yet class II-associated Ig-α/β appears to be derived from BCR. Hence, Ig-α/β are used in a sequential fashion for transduction of antigen and cognate T cell help signals.
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