630
UNIT FOUR
immunity—they stimulate B cells and secrete cytokines—it is
no wonder that harming them destroys immunity.
Another type of T cell is a
cytotoxic T cell,
which rec-
ognizes and combines with nonself antigens that cancerous
cells or virally infected cells display on their surfaces near
certain MHC proteins. Cytokines from helper T cells acti-
vate the cytotoxic T cell (F
g. 16.17
a
). Next the cytotoxic T
cell proliferates, enlarging its clone of cells. Cytotoxic T cells
then bind to the surfaces of antigen-bearing cells, where they
release
perforin
protein that cuts porelike openings, destroy-
ing these cells. In this way, cytotoxic T cells continually
monitor the body’s cells, recognizing and eliminating tumor
cells and cells infected with viruses.
Certain cytotoxic T cells, called CD8 T cells, give rise
to
memory T cells
that provide for future immune protec-
tion. When a CD8 T cell contacts an antigen-presenting cell,
it contorts into a dumbbell shape. The side of the dumbbell
that contacts the antigen-presenting cell accumulates differ-
ent receptors and other proteins from the side facing farthest
from the provoking antigen. When the CD8 T cell divides,
the daughter cell that was the part of the original cell clos-
est to the antigen becomes an active cytotoxic T cell. The
daughter cell farther from the antigen becomes a memory T
cell. As its name implies, a memory T cell does not respond
to an initial exposure to an antigen, but upon subsequent
exposure immediately divides and differentiates into a cyto-
toxic T cell. This response usually vanquishes the pathogen
before it can cause the body to produce signs and symptoms
of disease.
an
antigen-presenting cell
(accessory cell). Macrophages,
B cells, and several other cell types can be antigen-present-
ing cells.
T cell activation begins when a macrophage phago-
cytizes a bacterium, digesting it in its lysosomes. Some
bacterial antigens exit the lysosomes and move to the mac-
rophage’s surface. Here, they are displayed on the cell mem-
brane near certain protein molecules that are part of a group
of proteins called the
major histocompatibility complex
(MHC) or
human leukocyte antigens
(HLA)
because they
were F rst identiF ed on white blood cells. MHC antigens help
T cells recognize that an antigen is foreign, not self. Class I
MHC antigens are in cell membranes of all body cells except
red blood cells. Class II MHC antigens are on the surfaces of
antigen-presenting cells, thymus cells, and activated T cells.
Activated T cells interact directly with the antigen-
presenting cell. Such cell-to-cell contact is called the
cellu-
lar immune response,
or cell-mediated immunity.
T cells (and some macrophages) also synthesize and
secrete polypeptides called
cytokines
that enhance certain
cellular responses to antigens. ±or example,
interleukin-1
and
interleukin-2
stimulate synthesis of several cytokines
from other T cells. In addition, interleukin-1 helps activate
T cells, whereas interleukin-2 causes T cells to proliferate.
Other cytokines called
colony-stimulating factors
(CS±s) stim-
ulate production of leukocytes in the red bone marrow, cause
B cells to grow and mature, and activate macrophages. Cer-
tain cytokine combinations shut off the immune response.
Table 16.5
summarizes several cytokine types.
T cells may also secrete toxins that kill their antigen-
bearing target cells, growth-inhibiting factors that prevent
target cell growth, or interferon that inhibits the proliferation
of viruses and tumor cells. Several types of T cells have dis-
tinct functions.
A specialized type of T cell, called a
helper T cell,
becomes activated when its antigen receptor combines with
displayed foreign antigen
(f g. 16.17)
. Once activated, the
helper T cell stimulates the B cell to produce antibodies spe-
ciF c for the displayed antigen.
A type of helper T cell called a CD4 cell is the prime target
of HIV, the virus that causes AIDS. (CD4 stands for the “cluster-
of-differentiation” antigen it bears that enables it to recog-
nize a macrophage displaying a foreign antigen.) Considering
the role of CD4 helper T cells as key players in establishing
TABLE
16.4
|
A Comparison of T Cells and B Cells
Characteristic
T Cells
B Cells
Origin of undiF
erentiated cell
Red bone marrow
Red bone marrow
Site of diF
erentiation
Thymus
Red bone marrow
Primary locations
Lymphatic tissues, 70% to 80% of the circulating lymphocytes in
blood
Lymphatic tissues, 20% to 30% of the circulating lymphocytes
in blood
Primary functions
Provide cellular immune response in which T cells interact
directly with the antigens or antigen-bearing agents to destroy
them
Provide humoral immune response in which B cells interact
indirectly, producing antibodies that destroy the antigens or
antigen-bearing agents
TABLE
16.5
|
Types of Cytokines
Cytokine
Function
Colony-stimulating
factors
Stimulate bone marrow to produce lymphocytes
Interferons
Block viral replication, stimulate macrophages
to engulf viruses, stimulate B cells to produce
antibodies, attack cancer cells
Interleukins
Control lymphocyte diF
erentiation and growth
Tumor necrosis factor
Stops tumor growth, releases growth factors,
causes fever that accompanies bacterial infection,
stimulates lymphocyte diF
erentiation
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