FIGURE 23-32 Comparison of T-cell and B-cell development. Cell fate decisions are executed by receptors composed of either the newly rearranged µ chain (pre-BCR) or the newly rearranged β chain (pre-TCR). The pre-BCRs and pre-TCRs serve similar functions: signaling clonal expansion of cells that have successfully undergone rearrangement and allelic exclusion. This phase of lymphocyte development does not require antigen recognition. Both the pre-BCR and pre-TCR include subunits unique to each receptor type and absent from the antigen-specific receptors found on mature lymphocytes: VpreB and λ5 (orange, green) for the pre-BCR; pre-T α (blue) for the pre-TCR. Upon completion of the expansion phase, expression of the gene encoding the remaining subunit of the antigen-specific receptor begins: Ig light chain (light blue) for the BCR; TCR α chain (light red) for the TCR. Lymphocyte development and differentiation occur at distinct anatomic sites, and only fully assembled antigen-specific receptors (BCR, TCR) recognize antigen. Mature lymphocytes are strictly dependent on antigen recognition for their activation.