Page 298 - Veterinary Immunology, 10th Edition

time. A minimum of about 200 MHC class I molecules loaded with
VetBooks.ir the same viral peptide is required to activate a cytotoxic T cell. Thus

the MHC-peptide complexes can provide passing T cells with fairly
complete information on virtually all the proteins being made by a

cell. Analyses of peptide binding indicate that the binding groove of
a class I protein can bind over a million different peptides with
significant affinity. The number is not unlimited, however, because
each MHC type usually shows preferences for peptides with certain

structures. In fact, out of the great number of peptides generated
from the proteome of a pathogen, only a few “immunodominant”
peptides are recognized by most of the host's T cells. Thus cytotoxic
T cells can then screen these peptides to determine whether any are

“foreign” and bind to their TCRs.

Cross-Priming

It must not be assumed that the two antigen-processing pathways

function in isolation. In fact, the pathways interact extensively. For
example, under some circumstances, exogenous antigens may enter
the cytoplasm, join the endogenous antigen pathway, and be
presented on MHC class I molecules. Thus in antigen-presenting
cells such as macrophages and DCs, endocytosed viral antigen may

not be degraded in lysosomes but by proteasomes and so is
processed as an endogenous antigen. This antigen thus binds to
MHC class I molecules and is recognized by cytotoxic T cells. This

may be important in immunity to viruses since it ensures that the
antigens from dead virions may still be able to trigger a response by
cytotoxic T cells (Chapter 18).

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