FIGURE 14.9 • 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 that then enter the rough-surfaced endoplasmic reticulum (rER). In the rER, newly synthesized  chains of MHC I molecules interact with both the processed antigen and 2 microglobulin (2M) 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 site. 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 endocytosed, 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 lymphatic system to another at different stages in their development and to reach sites within the body where they are needed. Lymphocytes conveyed in the lymph enter lymph nodes via afferent lymphatic vessels, whereas lymphocytes conveyed in the blood enter the node through the walls of postcapillary venules (high endothelial venules [HEVs]; Fig. 14.11). B and T cells migrate to and populate different regions within the lymph node. Some lymphocytes pass through the substance of the node and leave via the efferent lymphatic vessels, which lead to the right lymphatic trunk or to the thoracic duct. In turn, both of these channels empty into the blood circulation at the junctions of the internal jugular and subclavian veins at the base of the neck. The lymphocytes are conveyed to and from the various lymphatic tissues via the blood vessels.
Diffuse lymphatic tissue and lymphatic nodules guard the body against pathogenic substances and are the site of the initial immune response.
The alimentary canal, respiratory passages, and genitourinary tract are guarded by accumulations of lymphatic
FIGURE 14.11 • Diagram depicting circulation of lymphocytes in the body. Lymphocytes enter lymph nodes by two routes: afferent lymphatic vessels and through the wall of high endothelial venules (HEVs) in the deep cortex. Some lymph node lymphocytes move to the T and B domains of the lymph node; others efferent pass through the parenchyma of the lymphatic vessel. Ultimately, the lym phocytes enter a major lymphatic vessel—in this case, the right lymphatic trunk—that opens into the junction of the right internal jugular and right subclavian vein. The lymphocytes continue to the arterial side of the circulation and, via the arteries, to the lymphatic tissues of the body or to tissues where they participate in immune reactions. From the lymphatic tissues, lymphocytes again return to the lymph nodes to gain entry via the HEV.