334
U N I T 4
Infection and Immunity
when they are presented with the foreign antigen pep-
tide associated with the class I MHC molecule. During
a typical viral infection of a cell, small peptides from
degraded viral proteins associate with MHC-I molecules
and are then transported to the infected cell membrane.
This complex communicates to the cytotoxic T cell that
the cell must be destroyed for the overall survival of the
host. The MHC-II molecule’s binding cleft accommo-
dates a fragment of antigen from pathogens that have
been engulfed and digested by an antigen-presenting cell
during the process of phagocytosis. The engulfed patho-
gen is degraded into peptides in cytoplasmic vesicles and
then complexed with the MHC-II molecule. Helper T
cells recognize these complexes on the surface of antigen-
presenting cells and become activated. These triggered
helper T cells multiply quickly and direct other immune
cells to respond to the invading pathogen through the
secretion of cytokines.
Each individual has a unique collection of MHC pro-
teins, and a variety of MHC molecules can exist in a pop-
ulation. Thus, MHC molecules are both polygenic and
polymorphic. The MHC genes are the most polymorphic
genes known. Because of the number of MHC genes and
the possibility of several alleles for each gene, it is almost
CD4
CD4
CD4
Virus
antigen
MHC-II
Antigenic
determinant
or epitope
TCR
TCR
Cytokines
MHC-I
with viral
epitope
Target
cell
TCR
CD8
CD8
CD8
Cell
death
Antibody
Plasma
cell
Memory
B
cell
B
cell
T-
helper
cell
T-helper
memory
cell
T-
helper
cell
Cytotoxic
T memory
cell
MHC-II
APC
Activation
Cytokines
TCR
Cytotoxic
T cell
FIGURE 15-7.
Pathway for immune cell participation in an adaptive immune response. APC, antigen-
presenting cell; MHC, major histocompatibility complex;TCR,T-cell receptor.
