Page 293 - Veterinary Immunology, 10th Edition

VetBooks.ir Antigen Processing

MHC Class II Pathway

The presentation of exogenous antigens is regulated by MHC class
II molecules. These are cell surface receptors that bind processed
peptide fragments. Although many cells can phagocytose foreign

antigens, only those that can express antigen fragments bound to
MHC class II molecules will trigger an immune response. As
described previously, the most efficient antigen-processing cells are
+
thus mature MHC class II cDCs. Unlike macrophages, dendritic
cell lysosomes have limited proteolytic activity and degrade

internalized antigens slowly. As a result, antigens may persist
within these cells for a long time. MHC class II molecules bind
fragments of these ingested antigens and present them to helper

cells (Fig. 10.10). T helper cells recognize and respond to antigen
fragments only when the fragments are bound to MHC class II
molecules. If an antigen is presented to T cells in the absence of
MHC class II, the T cells will be turned off or die, and tolerance may
result (Chapter 20).

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