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UNIT IV
Infection, Inflammation, and Immunity
longer period of time than their predecessors and as a result can
respond more rapidly on repeat exposure. The immune system
usually can respond to commonly encountered microorganisms
so efficiently that we are unaware of the response.
B and T lymphocyte activation is triggered by anti-
gen presentation to unique surface receptors (Fig. 13.6). The
antigen receptor present on the B lymphocyte consists of
membrane-bound Ig molecules that can bind a specific epitope.
However, in order for B lymphocytes to produce antibodies,
they require the help of specific T lymphocytes, called
helper
T cells
. While the B lymphocytes bind to one determinant (or
hapten) on an antigen molecule, the antigen-specific helper
T cell recognizes and binds to another determinant known as the
“carrier.” The carrier is an APC, which has previously picked
body, specific T cells become activated and specific B cells
are stimulated to produce antibodies. Once the first encounter
occurs, these cells can exactly recognize a particular microor-
ganism or foreign molecule because each lymphocyte is capa-
ble of targeting a specific antigen and differentiating the invader
from self or from other substances that may be similar to it.
Cell-mediated and humoral immunity is capable of respond-
ing to millions of antigens each day because there is an enor-
mous variety of lymphocytes that have been programmed and
selected during cellular development. Once the invading sub-
stance or organism has been removed from the body, the lym-
phocytes “remember” the presenting antigen and can respond
rapidly during the next encounter. These lymphocytes are called
“memory” T and B lymphocytes. They remain in the body for a
CD4
CD4
CD4
Virus
antigen
MHC-II
Antigen
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 13.6
•
Pathway for imm
une cell participation in an immune
response. (APC, antigen-presenting
cell; MHC, major histocompatibility
complex; TCR, T-cell receptor.)
