Page 295 - Veterinary Immunology, 10th Edition

endosomes, the foreign peptide fragments are exchanged for the
VetBooks.ir CLIP chain. An MHC class II antigen-binding groove can hold a

peptide of 12 to 24 amino acids as a straight, extended chain that
projects out of both ends of the binding groove. Side chains from

the peptide bind in pockets on the walls of the binding groove.
The presence of the CLIP chain prevents endosomes containing
MHC class II molecules from being prematurely transported to the
cell surface Thus, unlike most new transmembrane proteins that are

expressed minutes after assembly, MHC class II molecules are
retained inside the cell for several hours until they are needed.
Once the antigen peptide binds to an MHC molecule, the MIIC
vesicle moves toward the cell surface. When it reaches the surface,

the vesicle fuses with the cell membrane, and the MHC-peptide
complex is exposed and available for inspection by any passing T
cell.
It has been calculated that an antigen-processing DC contains

5
about 2 ×10 MHC class II molecules that can present peptide
fragments to T cells. If costimulation is provided, a single T cell can
be activated by exposure to as few as 200 to 300 of these peptide-
MHC complexes. It is therefore possible for a single antigen-

processing cell to present many different antigens to different T
cells simultaneously.
Since T helper cells must recognize MHC-antigen complexes in
order to respond to an antigen, the MHC class II molecules

effectively determine whether an animal will mount an adaptive
immune response to any antigen. Class II molecules can bind some,
but not all, peptides created during antigen processing, and, in
effect, they select those antigen fragments that are to be presented

to T cells. (Further coverage of MHC molecules is provided in
Chapter 11.)

MHC Class I Pathway

One function of T cell–mediated immune responses is the
identification and destruction of cells producing abnormal or
foreign proteins. The best examples of such cells are those infected
by viruses. Viruses take over the protein-synthesizing machinery of
infected cells and use it to make new viral proteins (Fig. 10.11). To

295