The specificity of tbe immune response is determined by the interaction between tbe B-cell, eceptor (BCR) and its cognate structure, antigen. Recent studies have provided considerable insigbt into the compartmentalization of function witbin this extremely versatile betero-oligomeric receptor complex. In tbis article, Cbristopber Pleiman, Daniele D'Ambrosio and] obn Cambier consolidate new findings regarding BCR structure and signal transduction.

The response of B cells to pathogens and other foreign substances is mediated, in part, through the cell-surface B-cell receptor (BCR) for antigen. Stimulation of this receptor by paucivalent proteinacious antigens leads to antigen uptake and processing, as well as the priming of cells to present antigen to T cells 1. Signaling through this receptor can also lead to more-profound biological responses, including activation, tolerance and/or differentiation, depending on the nature of the stimulus and the differentiative state of the B cell 2-4. Recent studies have shown that the BCR belongs to a class of receptors that includes the T-cell receptor (TCR), as well as receptors for the Fc region of IgE (Fc¢ RI) and IgG (FcR'ylIIa)(reviewed in Ref. 5). These receptors are characterized by a complex hetero-oligomeric structure in which ligand binding and signal transduction are compartmentalized into distinct receptor subunits. In the BCR, the! igand-binding portion of the receptor is surface immunoglobulin (sIg), which is a tetrameric complex of Ig heavy (H) chains and light (L) chains. The sIg-associated'transducer'structure comprises a disulfide-bonded heterodimer (s) of Ig-ct and Ig-[3, which were recently designated CD79a and CD79b, respectively. These proteins are structurally related and are products of the Ig superfamily genes rob-1 and B29,.~ nd function in receptor transport and signal transduction. This review will focus on recent progress in the understanding of the BCR structure and signal transduction.