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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.
| 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. |
| 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.
| 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.
1-26-05 repost |