The Body's First
Line of Defense
The immune system is a complex of organs--highly specialized
cells and even a circulatory system separate from blood
vessels--all of which work together to clear infection from
the body.
The organs of the immune system, positioned throughout
the body, are called lymphoid organs. The word
"lymph" in Greek means a pure, clear stream--an
appropriate description considering its appearance and
purpose.
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Lymphatic
vessels form a circulatory system that
operates in close partnership with blood
circulation. |
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Lymphatic vessels and lymph nodes are
the parts of the special circulatory system that
carries lymph, a transparent fluid containing white
blood cells, chiefly lymphocytes. |
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Lymph bathes the tissues
of the body, and the lymphatic vessels collect and
move it eventually back into the blood circulation.
Lymph nodes dot the network of lymphatic vessels and
provide meeting grounds for the immune system cells
that defend against invaders. The spleen, at the
upper left of the abdomen, is also a staging ground
and a place where immune system cells confront
foreign microbes. |
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| Organs
and tissues of the immune system dot the
body in a protective network of barriers to
infection. |
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Pockets of lymphoid tissue are in many other locations
throughout the body, such as the bone marrow and thymus.
Tonsils, adenoids, Peyer's patches, and the appendix are
also lymphoid tissues.
Both immune cells and foreign molecules enter the lymph
nodes via blood vessels or lymphatic vessels. All immune
cells exit the lymphatic system and eventually return to the
bloodstream. Once in the bloodstream, lymphocytes are
transported to tissues throughout the body, where they act
as sentries on the lookout for foreign antigens.
How the Immune
System Works
Cells that will grow into the many types of more specialized
cells that circulate throughout the immune system are
produced in the bone marrow. This nutrient-rich, spongy
tissue is found in the center shafts of certain long, flat
bones of the body, such as the bones of the pelvis. The
cells most relevant for understanding vaccines are the
lymphocytes, numbering close to one trillion.
The two major classes of lymphocytes are B cells, which
grow to maturity in the bone marrow, and T cells, which
mature in the thymus, high in the chest behind the
breastbone.
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B cells produce antibodies that circulate in the
blood and lymph streams and attach to foreign
antigens to mark them for destruction by other
immune cells.
B cells are part of what is known as
antibody-mediated or humoral immunity, so called
because the antibodies circulate in blood and lymph,
which the ancient Greeks called, the body's
"humors."
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B cells become plasma cells, which
produce antibodies when a foreign antigen
triggers the immune response. |
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Certain T cells, which also patrol the blood and lymph
for foreign invaders, can do more than mark the antigens;
they attack and destroy diseased cells they recognize as
foreign. T lymphocytes are responsible for cell-mediated
immunity (or cellular immunity). T cells also orchestrate,
regulate and coordinate the overall immune response. T cells
depend on unique cell surface molecules called the major
histocompatibility complex (MHC) to help them recognize
antigen fragments.
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Antibodies
produced by cells of the immune system
recognize foreign antigens and mark them for
destruction. |
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Antibodies
The antibodies that B cells produce are basic
templates with a special region that is highly
specific to target a given antigen. Much like a car
coming off a production line, the antibody's frame
remains constant, but through chemical and cellular
messages, the immune system selects a green sedan, a
red convertible or a white truck to combat this
particular invader.
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However, in contrast to cars, the variety of antibodies
is very large. Different antibodies are destined for
different purposes. Some coat the foreign invaders to make
them attractive to the circulating scavenger cells,
phagocytes, that will engulf an unwelcome microbe.
When some antibodies combine with antigens, they activate
a cascade of nine proteins, known as complement, that have
been circulating in inactive form in the blood. Complement
forms a partnership with antibodies, once they have reacted
with antigen, to help destroy foreign invaders and remove
them from the body. Still other types of antibodies block
viruses from entering cells.
T Cells
T cells have two major roles in immune defense. Regulatory T
cells are essential for orchestrating the response of an
elaborate system of different types of immune cells.
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Helper T cells, for example, also known as CD4
positive T cells (CD4+ T cells), alert B cells to
start making antibodies; they also can activate
other T cells and immune system scavenger cells
called macrophages and influence which type of
antibody is produced.
Certain T cells, called CD8 positive T cells
(CD8+ T cells), can become killer cells that attack
and destroy infected cells. The killer T cells are
also called cytotoxic T cells or CTLs (cytotoxic
lymphocytes).
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T
lymphocytes become CD4+ or helper T cells,
or they can become CD8+ cells, which in turn
can become killer T cells, also called
cytotoxic T cells. |
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Immune system
process
Activation of helper
T cells
After it engulfs and processes an antigen, the macrophage
displays the antigen fragments combined with a Class II MHC
protein on the macrophage cell surface. The antigen-protein
combination attracts a helper T cell, and promotes its
activation.
Activation of
cytotoxic T cells
After a macrophage engulfs and processes an antigen, the
macrophage displays the antigen fragments combined with a
Class I MHC protein on the macrophage cell surface. A
receptor on a circulating, resting cytotoxic T cell
recognizes the antigen-protein complex and binds to it. The
binding process and a helper T cell activate the cytotoxic T
cell so that it can attack and destroy the diseased cell.
Activation of B
cells to make antibody
A B cell uses one of its receptors to bind to its
matching antigen, which the B cell engulfs and processes.
The B cell then displays a piece of the antigen, bound to a
Class II MHC protein, on the cell surface. This whole
complex then binds to an activated helper T cell. This
binding process stimulates the transformation of the B cell
into an antibody-secreting plasma cell. |
Therapeutic
Dosage
