664
Chapter 18
and vigorously should the antigen reappear at a future time
(see Figure 18–14).
The example we have been using employed a macro-
phage as the APC to helper T cells, but B cells can also serve
in this role (see Figure 18–11). The binding of the helper T
cell to the antigen-bound B cell ensures maximal stimulation
of the B cell by the cytokines secreted by that helper T cell and
any of its progeny that remain nearby.
Antibody Secretion
After their differentiation from B cells, plasma cells produce
thousands of antibody molecules per second before they die
in a day or so. We mentioned earlier that there are fi ve major
classes of antibodies. The most abundant are the
IgG
antibod-
ies, commonly called
gamma globulin,
and
IgM
antibodies.
These two groups together provide the bulk of specifi c immu-
nity against bacteria and viruses in the extracellular fl
uid.
IgE
antibodies participate in defenses against multicellular para-
sites and also mediate allergic responses.
IgA
antibodies are
secreted by plasma cells in the linings of the gastrointestinal,
respiratory, and genitourinary tracts; these antibodies gener-
ally act locally in the linings or on their surfaces. They are also
secreted by the mammary glands and therefore are the major
antibodies in milk. The functions of
IgD
are still unclear.
In the kind of infection described in this chapter, the B
and plasma cells, sitting on the nodes near the infected tissues,
recognize antigen and are activated to make antibodies. The
antibodies (mostly IgG and IgM) circulate through the lymph
and blood to return to the infected site. At sites of infection,
the antibodies leave the blood (recall that nonspecifi c infl
am-
+
+
+
Bacterium
Skin surface
Open
cut
Blood or lymphatic vessel
Class II MHC protein
Processed antigen
Macrophage
B cell
Lymph node
or spleen
Helper
T cell
IL-1, TNF
IL-2
IL-2 and other
cytokines
Plasma
cells
Specific
antibodies
Secreted
into systemic
circulation
Memory cell
End
Begin
Figure 18–14
Summary of events by which a bacterial infection leads to antibody synthesis in peripheral lymphoid organs. The secreted antibodies travel by
the blood to the site of infection, where they bind to bacteria of the type that induced the response. The attack triggered by antibody’s binding
to bacteria is described in the text. (As illustrated in Figure 18–11b, an antigen-bound B cell, rather than a macrophage, as shown in this fi
gure,
can function as the antigen-presenting cell to the helper T cell. Also for clarity, the intracellular processing of the antigen by the macrophage
[see Figure 18–11a] is not shown in this fi
gure.)
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