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 It molecules to associate with Immunoglobulin (Ig)-alpha /Ig-beta (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-alpha /Ig-beta are distinct complexes, yet class It-associated Ig-alpha/beta appears to be derived from BCR. Hence, Ig-alpha/beta are used in a sequential fashion for transduction of antigen and cognate T cell help signals.