Figure 1310

Schematic diagram of processing pathways for MHC I and MHC II synthesis and antigen presentation. During the processing and presentation of cytoplasmic antigen (Ag) for MHC I molecules (red pathway), cytoplasmic protein antigens are degraded by protease into 8 to 10 amino acid fragments, which then enter the rER. In the rER, newly synthesized a chains of MHC I molecules interact with both the processed antigen and f32 microglobulin (fi2M) and form a stable complex. This complex leaves the rER via the typical secretory pathway through the Golgi apparatus. The antigen-MHC I complex is displayed on the cell surface, where it is available for recognition by cytotoxic CD8+ T lymphocytes. MHC II molecules are assembled in the rER and then bind to an invariant chain, which blocks the antigen-

binding side. At this point the MHC II molecule and the invariant chain are secreted to the cell surface (blue pathway). After a brief stay on the cell surface, the MHC II molecule and invariant chain are en-docytosed, and in an early endosome, the invariant chain is degraded. The foreign (exogenous) antigen is endocytosed and partially digested by proteolytic degradation in endosomes (white pathway). The MHC II molecule can now bind the processed foreign antigen and return with it to the cell surface. On the cell surface, the antigen-MHC II complex is recognized by helper CD4+ T lymphocytes, which initiates the immune response. If the MHC II molecule fails to capture the antigen, it will be degraded in the lysosomal compartment (green pathway).

the epithelium of skin and mucous membranes. These vessels remove substances and fluid from the extracellular spaces of the connective tissues, thus producing lymph. Because the walls of the lymphatic capillaries are more permeable than the walls of blood capillaries, large molecules, including antigens and cells, gain entry more readily into the lymphatic capillaries than into blood capillaries.

As lymph circulates through the lymphatic vessels, it passes through lymph nodes. Within the lymph nodes, foreign substances (antigens) conveyed in the lymph are trapped by the follicular dendritic cells and concentrated. These APCs process the antigens and present them to lymphocytes, which leads to an immune response that eliminates the antigens from the body.

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