The Immune System: Innate and Adaptive Body Defenses
ﬂags” (class I MHC proteins
NK cell attack), whereas T
cells check the “identity ﬂags” to see if they look the way they are
supposed to (foreign antigens
Regulatory T Cells
cells help activate adaptive immune responses, related
T cells called
regulatory T (T
dampen the immune re-
sponse. ±ey act either by direct contact or by releasing inhibi-
tory cytokines such as IL-10 and TGF-β.
cells are important in preventing autoimmune reactions
because they suppress self-reactive lymphocytes in the periphery—
that is, outside the lymphoid organs. T
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.
on p. 792 summarizes the
apparently the T
cells sometimes “see” the foreign class I MHC
antigens as a combination of self class I MHC protein bound to
Once cytotoxic T cells recognize their targets, how do they
? ±ere are two major mechanisms. One in-
. ±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
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
cells can’t “see.”
NK cells and T
lymphocytes roam the body, adhering to
and crawling over the surfaces of other cells, examining them
for markers they might recognize, a process called
. NK cells check to make sure each cell has “identity
on MHC I
binds tightly to
Perforin molecules insert into
the target cell membrane,
polymerize, and form
transmembrane pores (cylindrical
holes) similar to those produced
by complement activation.
Granzymes enter the
target cell via the pores.
Once inside, granzymes
activate enzymes that
and searches for
(a) A mechanism of target cell killing by T
its granules by
T cell (T
(b) Scanning electron micrograph of a
cell killing a cancer cell (2100
Cytotoxic T cells attack infected and cancerous cells.
MHC proteins and
T cell receptors not shown.