Figure 134

Schematic diagram of the molecular structure of MHC I and MHC II molecules.The MHC I molecule is a glycoprotein that is expressed on the surface of all nucleated cells of the body and on platelets. MHC I molecules present endogenously synthesized peptides for recognition by cytotoxic CD8+ T lymphocytes. Therefore, the MHC I molecule acts as the target for the elimination of abnormal host cells producing abnormal proteins (e.g., cells infected by an intracellular agent, such as a virus, or cells that have been transformed, such as cancer cells). MHC I consists of an a heavy chain (45 kDa) and a smaller, nonco-

valently attached /32 microglobulin polypeptide (12 kDa). The fi2 microglobulin promotes maturation of T cells and acts as a chemotactic factor. The MHC II molecule is also a glycoprotein but is expressed only on a restricted population of cells known as antigen-presenting cells (APCs). MHC II molecules present exogenous (foreign) peptides to helper CD4+ T lymphocytes. They consist of two chains: an « chain (33 kDa) and a /3 chain (29 kDa), each of which possesses oligosaccharide groups.

the class of the MHC molecule engaged. This restricted presentation of foreign antigens by MHC molecules to either cytotoxic or helper T lymphocytes is a key component of immune surveillance.

The MHC I molecule with the peptide antigen displayed on its surface interacts only with the TCR and CD8+ molecule expressed on cytotoxic CD8+ T lymphocytes; these cells are therefore described as MHC I restricted. This interaction allows cytotoxic T lymphocytes to recognize infected or transformed target cells (Fig. 13.5a).

In ( jntrast, the MHC II molecule with the peptide antigen displayed on its surface interacts only with the TCR and CD4 molecule expressed on helper CD4+ T lymphocytes (Fig. 13.5b); these cells are therefore described as MHC II restricted. MHC II molecules are found on APCs, such as macrophages, whose main function is to present antigen to T lymphocytes.

The humoral immune response: Activated B lymphocytes differentiate into plasma cells that produce antibodies or into B memory cells

Each B lymphocyte reacts only with a single antigen or type of antigenic site that it has been genetically programmed to recognize. The reaction of a B lymphocyte with a TCR-MITC II—antigen complex activates the cell. Details of B cell activation are illustrated in Fig. 13.6. Activated B lymphocytes are transformed into immnnoblasts (plasmablasts) that proliferate and then differentiate into

• Plasma cells, which synthesize and secrete a specific antibody

• Memoiy B cells, which respond more quickly to the next encounter with the same antigen

The specific antibody produced by the plasma cell binds to the stimulating antigen, forming an antigen-antibody


Schematic diagram of the molecular interactions that occur during antigen presentation. To become activated, both cytotoxic and helper T lymphocytes need to identify presented antigen as "nonself" as well as recognize the appropriate class of MHC molecules, a. In all nucleated cells of the body, viral antigen or cancer (tumor-specific)

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