The series of stages leading to T-cell-dependent B-cell activation in the T zones is shown in Figure 20.11. T lymphocytes entering the T zone 'inspect' peptides held in MHC molecules of local IDCs and this interaction involves intimate contact between the two cells. If a T cell is able to recognize a peptide-MHC complex, it is activated through TCR signalling. Costimulation is provided by interaction between molecules such as CD80 and CD86 expressed on the IDCs and CD28, which is constitutively expressed by T cells. The effect of this interaction is to bring about changes in the T cell that are collectively called T-cell priming. These include the ability to express CD40 ligand and CTLA-4, a high-affinity ligand for CD80 and CD86. B cells cannot prime virgin T cells but are able to interact with primed T cells in the T zone (Figure 20.12).
Cytokines are produced by the T cell following this interaction and the nature of the cytokines produced in different situations is considered later. Short-term proliferation of the T and B cells is also induced, and most B cells migrate to local sites of antibody production. In the spleen this is the red pulp, and in lymph nodes the medullary cords. The lifespan of most of these plasma cells is 3 days. The extent of immunoglobulin class switching will depend on the conditions of dendritic cell activation and T-cell priming. In primary antibody responses, the plasma cells generated by B-cell activation in T zones do not have somatic mutations in their Ig V-region genes. On the other hand, in secondary responses, marginal-zone memory B cells that have somatic mutations in their V region genes can be induced to migrate to T zones on contact with antigen and give rise to short-lived plasma cells.
The other pathway of migration of T and B cells activated in T zones is to the follicles. Both antigen-specific B blasts and T blasts migrate to the follicles at an early stage in T-zone responses and give rise to germinal centres. This process is described in the section on follicular responses.
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