Chapter 21
The Immune System: Innate and Adaptive Body Defenses
789
21
flags” (class I MHC proteins
inhibit
NK cell attack), whereas T
C
cells check the “identity flags” to see if they look the way they are
supposed to (foreign antigens
stimulate
T
C
cell attack).
Regulatory T Cells
While T
H
cells help activate adaptive immune responses, related
T cells called
regulatory T (T
Reg
) cells
dampen the immune re-
sponse. ±ey act either by direct contact or by releasing inhibi-
tory cytokines such as IL-10 and TGF-β.
T
Reg
cells are important in preventing autoimmune reactions
because they suppress self-reactive lymphocytes in the periphery—
that is, outside the lymphoid organs. T
Reg
cells and their subpopula-
tions are currently hot research topics. For example, researchers
hope to use them to induce tolerance to transplanted tissue and to
lessen the severity of autoimmune diseases.
■ ■ ■
In summary, each type of T cell has unique roles to play in the
immune response, yet is intimately intertwined with other im-
mune cells and elements.
Table 21.7
on p. 792 summarizes the
apparently the T
C
cells sometimes “see” the foreign class I MHC
antigens as a combination of self class I MHC protein bound to
foreign antigen.
Once cytotoxic T cells recognize their targets, how do they
deliver a
lethal hit
? ±ere are two major mechanisms. One in-
volves
perforins
and
granzymes
(Figure 21.19)
. ±e other
involves binding to a specific membrane receptor on the target
cell that stimulates the target cell to undergo apoptosis.
NK cells, introduced earlier, use the same key mechanisms to
kill their target cells. NK cells, however, do not look for foreign
antigen displayed on class I MHC proteins. Instead they search
for other signs of abnormality, including the
lack
of class I MHC
or the presence of antibodies coating the target cell. Stressed
cells also o²en express different surface markers (such as some
belonging to a family called MIC), which can activate NK cells.
In short, NK cells stalk abnormal or foreign cells in the body
that T
C
cells can’t “see.”
NK cells and T
C
lymphocytes roam the body, adhering to
and crawling over the surfaces of other cells, examining them
for markers they might recognize, a process called
immune
surveillance
. NK cells check to make sure each cell has “identity
1
T
C
identifies
foreign antigens
on MHC I
proteins and
binds tightly to
target cell.
3
Perforin molecules insert into
the target cell membrane,
polymerize, and form
transmembrane pores (cylindrical
holes) similar to those produced
by complement activation.
4
Granzymes enter the
target cell via the pores.
Once inside, granzymes
activate enzymes that
trigger apoptosis.
5
The T
C
detaches
and searches for
another prey.
(a) A mechanism of target cell killing by T
C
cells.
2
T
C
releases
perforin
and
granzyme
molecules from
its granules by
exocytosis.
Cytotoxic
T cell (T
C
)
(b) Scanning electron micrograph of a
T
C
cell killing a cancer cell (2100
m
).
Cytotoxic
T cell
Cancer cell
Target
cell
Perforin
T
C
cell
membrane
Target
cell
membrane
Perforin
pore
Granzymes
Granule
Adaptive defenses
Cellular immunity
Figure 21.19
Cytotoxic T cells attack infected and cancerous cells.
MHC proteins and
T cell receptors not shown.
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