One of the most puzzling features of the
antibody response is how an antigenspecific
B cell manages to encounter a helper T cell with an appropriate
antigen specificity. This question arises because the frequency of naive
lymphocytes specific for any given antigen is estimated to be between 1 in 10,000 and 1 in 1,000,000. Thus, the chance of an encounter between a T lymphocyte and a
B lymphocyte that recognize the same antigen should be between 1 in 10[SUP]8[/SUP] and 1 in 10[SUP]12[/SUP]. Achieving such an encounter is a far more difficult challenge than getting effector
T cells activated, because, in the latter case, only one of the two cells involved has specific receptors. Moreover, T cells and B cells mostly occupy quite distinct zones in peripheral lymphoid tissue (see
Fig. 1.8). As in naive T-cell activation (see Chapter 8), the answer seems to lie in the antigen-specific trapping of migrating lymphocytes.
When an
antigen is introduced into an animal, it is captured and processed by professional antigen-presenting cells, especially the dendritic cells that migrate from the tissues into the
T-cell zones of local lymph nodes. Recirculating naive
T cells pass by such cells continuously and those rare T cells whose receptors bind peptides derived from the antigen are trapped very efficiently. This trapping clearly involves the specific antigen receptor on the T cell,