788
UNIT 4
Maintenance of the Body
21
T
H
1 cells
stimulate inflammation, activate macrophages, and
promote differentiation of cytotoxic T cells. In other words,
they mediate most aspects of cellular immunity.
T
H
2 cells
mainly defend against parasitic worms. ±ey mo-
bilize eosinophils to the battlefield and activate immune re-
sponses that depend on B cells and antibody formation. ±ey
are also the cells that promote allergies.
T
H
17 cells
link together adaptive and innate immunity by
releasing IL-17, which promotes inflammatory responses
against extracellular microbes and may underlie most au-
toimmune diseases.
Cytotoxic T Cells
Cytotoxic T (T
C
) cells
(activated CD8 cells) are the only T
cells that can directly attack and kill other cells. T
C
cells roam
the body, circulating in and out of the blood and lymph and
through lymphoid organs in search of body cells displaying
antigens that the T
C
cells recognize. ±eir main targets are
virus-infected cells, but they also attack tissue cells infected
by certain intracellular bacteria or parasites, cancer cells, and
foreign cells introduced into the body by blood transfusions or
organ transplants.
Before the onslaught can begin, the cytotoxic T cell must
“dock” on the target cell by binding to a self-nonself complex.
Remember, all body cells display class I MHC antigens, so T
C
cells can destroy all infected or abnormal body cells. ±e attack
on foreign human cells, such as those of a graF, is more difficult
to explain because here
all
of the antigens are nonself. However,
cell signals for antibody formation to begin. B cells continue to
divide as long as T
H
stimulation continues. In this way, helper T
cells help unleash the protective potential of B cells.
B cells may be activated solely by binding to certain anti-
gens called
T cell–independent antigens
. However, T cell-
independent antigen responses tend to be weak and short-lived.
Most antigens are
T cell–dependent antigens
that require T cell
help to activate the B cells to which they bind.
Activation of CD8 Cells
Like B cells, CD8 cells usually require
help from T
H
cells to activate into destructive cytotoxic T cells.
As shown in ²igure 21.18b, T
H
cells cause dendritic cells to ex-
press on their surfaces the co-stimulatory molecules required to
activate CD8 cells.
Amplification of Innate Defenses
T
H
cells also amplify the
responses of the innate immune system. ²or example, they acti-
vate macrophages to become more potent killers. ±e cytokines
released by T
H
cells not only mobilize lymphocytes and macro-
phages but also attract other types of white blood cells into the
area and tremendously amplify innate defenses. As the released
chemicals summon more and more cells into the battle, the im-
mune response gains momentum, and the sheer number of im-
mune elements overwhelms the antigens.
Subsets of T
H
Cells
It is interesting, but not surprising, that
different subsets of helper T cells exist. ±e subset that develops
during T
H
cell differentiation depends on the type of antigen
and the site at which it is encountered as well as the cytokine
exposure of the differentiating T
H
cell.
Class II MHC
protein
Class I
MHC protein
APC (dendritic cell)
IL-2
CD4 protein
CD8 T cell
(becomes T
C
cell
after activation)
Helper T cell
CD8
protein
B cell (being activated)
MHC II protein
of B cell displaying
processed antigen
IL-4 and other
cytokines
Helper T cell
CD4 protein
T cell receptor (TCR)
Helper T cell
Adaptive defenses
Humoral immunity
Cellular immunity
1
2
T
H
cell binds
with the self-nonself
complexes of a B cell
that has encountered
its antigen and is
displaying it on
MHC II on its surface.
T
H
cell releases
interleukins as co-
stimulatory signals to
complete B cell
activation.
1
2
T
H
cell binds
dendritic cell.
T
H
cell
stimulates dendritic
cell to express
co-stimulatory
molecules.
3
Dendritic cell
can now activate
CD8 cell with the
help of interleukin 2
secreted by T
H
cell.
(a)
Helper T cells help in humoral immunity
(b)
Helper T cells help in cellular immunity
Figure 21.18
The central role of helper
T cells in mobilizing both humoral and
cellular immunity.
(a)
T
H
and B cells usually
must interact directly for full B cell activation.
(b)
In response to T
H
cell binding, dendritic cells
express co-stimulatory molecules required to
activate CD8 T cells. (Some types of antigens
induce these co-stimulatory molecules
themselves, in which case T
H
cell help may
not be needed.) The T
H
cell also produces
interleukin 2, which causes the CD8 cell to
proliferate and differentiate into a T
C
cell.
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