Interaction Between Antigen Presenting Cells and TLymphocytes

Linear peptide fragments bind to the peptide groove of MHC molecules in the rough endoplasmic reticulum of antigen presenting cells and then these molecules are expressed on the cell surface. Only a minority of peptide fragments from a protein antigen (from any source) are capable of being processed in this way.

The antigen is "recognized' by specific T-helper cells (TH). The T cell receptor (TCR) binds to the antigen-MHC molecule complex. Further molecular interactions occur once the cells are brought into close proximity by this interaction (Figure HA.24). The result is an exchange of cytokines including interleukin 1 (IL-1), produced by macrophages, which stimulates TH proliferation and expression of interleukin 2 (IL-2) receptors on TH-cells. Activation of T-cells also results in the production of interferon y that stimulates the expression of MHC molecules on macrophages, producing positive feedback.

These processes are the first events following challenge by a new antigen and the development of an effective immune response depends on the production, by proliferation, of adequate numbers of TH-cells. An

Prolifeiotion of T helper cells

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Figure HA.24 Interaction between antigen presenting cell and helper T cell immune response follows and this can be antibody-mediated (a B lymphocyte function) or cell-mediated (a T cell and macrophage function).

Antibody-Mediated Response

Antigen binds to the surface immunoglobulin of specific B lymphocytes. It is processed and expressed in association with MHC molecules. The B cell is now capable of interacting with specific TH-cells, which will have been activated by interaction with APCs. This B-T interaction results in delivery of stimulatory cytokines by the TH-cells, the result is the proliferation and then differentiation of B cells to produce plasma cells and then antibody (Figure HA.25). B cells can also be activated by interacting with antigen presented by APCs, with subsequent proliferation and differentiation signals coming from TH-cells. A proportion of B-cells, rather than differentiating to form plasma cells, enters a resting phase to become memory B-cells (BM). Similarly a subpopulation of the stimulated TH population will become TH-memory cells.

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