186
UNIT 3
Organ Systems
Figure 10-5A.
A representation of helper T-lymphocyte activation.
The
CD4
surface marker on a
helper T cell
recognizes
MHC II
surface proteins on the
antigen-presenting cell
. The
TCR
binds with the
peptide-MHC complex on the surface of the macrophage (or other types of antigen-presenting cells); therefore, antigens are presented
to helper T cells. The activating signals (secreted proteins, cytokines) are exchanged between the helper T cells and the macrophages.
There are two main types of helper T cells: (1) Activated
helper (T
H
2) cells
release a variety of
interleukins
/
cytokines
that stimulate
B cells
to proliferate and increase the population of plasma cells, thereby increasing production of
antibodies
. (2) Activated helper
(T
H
1) cells
release and bind with
IL-2,
stimulating proliferation and activation of T
H
1 cells and greatly increasing their own numbers.
Activated T
H
1 cells provide signals that promote proliferation of
cytotoxic T cells
(CD8
+
cells) and activation of
macrophages
. In turn,
activated macrophages kill bacteria by a variety of mechanisms (see Fig. 8-6) and stimulate additional infl ammatory processes.
M
M
M
MHC II
MHC II
Antigen
(1) Helper
(T 2) cells
H
(2) Helper
(T 1) cells
H
B cells
T 1 cells
H
T 1 cells
H
T 1 cells
H
Plasma cell
Antibodies
CD4
CD4
TCR
TCR
IL-2
IL-4, IL-5, IL-6
IFN-„
Antigen-presenting
cell (macrophage)
Antigen-presenting
cell (macrophage)
Activated macrophage
(kill bacteria)
A
CLINICAL CORRELATION
Figure 10-5B.
Human Immunode±
ciency Virus Infection.
Infection by the
retrovirus HIV
leads to
acquired immunode± ciency
syndrome
(
AIDS
). Infection may be transferred from an infected indi-
vidual through exposure to body fl uids including blood, semen, and
breast milk. It is associated with a progressive
decline
in
CD4
+
T cell
numbers. The stage of infection can be determined by measuring the
patient’s CD4
+
T cell number and the level of HIV in the blood. HIV
primarily infects CD4
+
helper T cells, macrophages, and dendritic cells
(antigen-presenting cells). The low level of CD4
+
T cells in the blood
of HIV-infected patients may be because of (1) the HIV virus killing
infected CD4
+
T cells directly, (2) increased rates of apoptosis in infected
CD4
+
T cells, or (3) CD8
+
cytotoxic lymphocytes recognizing and kill-
ing CD4
+
T cells after the virus has infected them. The HIV virus enters
macrophages (CD4
+
T cells as well), replicates in the host cells, and the
new viruses are released from the host cells. Greatly reduced numbers
of CD4
+
T cells result in the loss of cell-mediated immunity. Without
stimulation from CD4
+
T helper cells, humoral immunity function is
compromised. AIDS patients are vulnerable to opportunistic infections;
common diseases include
Pneumocystis jiroveci pneumonia
,
toxoplas-
mosis
, and
thrush
. Histologically, lymph nodes in the early stage of HIV
infection reveal large, irregular lymphatic nodules and an increased
number of macrophages in the germinal centers (Fig. 10-9C).
Perforins
CD4
TCR
Helper
T cells
Infected
macrophage
Macrophage
HIV
CD8
TCR
Cytotoxic T cells
B
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