Key Concepts of Section 23.6

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Key Concepts of Section 23.6

Collaboration of Immune-System Cells in the Adaptive Response

  • Antigen-presenting cells such as dendritic cells require activation by means of signals delivered to their Toll-like receptors. These receptors are broadly specific for macromolecules produced by bacteria and viruses. Engagement of TLRs activates the NF-κB signaling pathway, whose outputs include the synthesis of inflammatory cytokines (see Figure 23-35).

  • Upon activation, dendritic cells become migratory and move to lymph nodes, ready for their encounter with T cells. Activation of dendritic cells also increases their display of MHC-peptide complexes and expression of co-stimulatory molecules required for initiation of a T-cell response.

  • B cells require the assistance of T cells to execute their full differentiation program to become plasma cells. Antigen-specific help is provided to B cells by activated T cells, which recognize class II MHC–peptide complexes on the surfaces of B cells. These B cells generate the relevant MHC-peptide complexes by internalizing antigen via BCR-mediated endocytosis, followed by antigen processing and presentation via the class II MHC pathway (see Figure 23-37).

  • In addition to cytokines produced by activated T cells, B cells require cell-cell contact to initiate somatic hypermutation and class-switch recombination. This contact involves CD40 on B cells and CD40L on T cells.

  • Important applications of the immunological concept of collaboration between T and B cells include vaccines. The most common forms of vaccines are live attenuated viruses or bacteria, which can evoke a protective immune response without causing pathology, and subunit vaccines.

  • The adaptive immune system can sometimes distinguish between normal cells and their cancerous counterparts. What complicates immune-system detection of cancer cells are the often relatively minor differences between normal and transformed cells.

  • Immunological checkpoints dampen the activity of antigen-specific T cells, under normal circumstances as a means of turning off or controlling an immune response.