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Understanding the Immune
System
The Anatomy of the Immune
System
The organs of the immune system
are stationed throughout the body. They are generally referred to as lymphoid
organs because they are concerned with the growth, development, and deployment
of lymphocytes, the white cells that are the key operatives of the immune
system. Lymphoid organs include the bone marrow and the thymus, as well as lymph
nodes, spleen, tonsils and adenoids, the appendix, and clumps of lymphoid tissue
in the small intestine known as Peyer's patches. The blood and lymphatic vessels
that carry lymphocytes to and from the other structures can also be considered
lymphoid organs.
Cells destined to become immune cells,
like all other blood cells, are produced in the bone marrow, the soft tissue in
the hollow shafts of long bones. The descendants of some so-called stem cells
become lymphocytes, while others develop into a second major group of immune
cells typified by the large, cell-and particle-devouring white cells known as
phagocytes.
The two major classes of lymphocytes are B
cells and T cells. B cells complete their maturation in the bone marrow. T
cells, on the other hand, migrate to the thymus, a multilobed organ that lies
high behind the breastbone. There they multiply and mature into cells capable of
producing immune response-that is, they become immunocompetent. In a process
referred to as T cell "education," T cells in the thymus learn to
distinguish self cells from nonself cells; T cells that would react against self
antigens are eliminated.
Upon exiting the bone marrow and thymus,
some lymphocytes congregate in immune organs or lymph nodes. Others-both B and T
cells-travel widely and continuously throughout the body. They use the blood
circulation as well as a bodywide network of lymphatic vessels similar to blood
vessels.
Laced along the lymphatic routes-with
clusters in the neck, armpits, abdomen, and groin-are small, bean-shaped lymph
nodes. Each lymph node contains specialized compartments that house platoons of
B lymphocytes, T lymphocytes, and other cells capable of enmeshing antigen and
presenting it to T cells. Thus, the lymph node brings together the several
components needed to spark an immune response.
The spleen, too, provides a meeting ground
for immune defenses. A fist-sized organ at the upper left of the abdomen, the
spleen contains two main types of tissue: the red pulp that disposes of worn-out
blood cells and the white pulp that contains lymphoid tissue. Like the lymph
nodes, the spleen's lymphoid tissue is subdivided into compartments that
specialize in different kinds of immune cells. Microorganisms carried by the
blood into the red pulp become trapped by the immune cells known as macrophages.
(Although people can live without a spleen, persons whose spleens have been
damaged by trauma or by disease such as sickle cell anemia, are highly
susceptible to infection; surgical removal of the spleen is especially dangerous
for young children and the immunosuppressed.)
Nonencapsulated clusters of lymphoid
tissue are found in many parts of the body. They are common around the mucous
membranes lining the respiratory and digestive tracts-areas that serve as
gateways to the body. They include the tonsils and adenoids, the appendix, and
Peyer's patches.
The lymphatic vessels carry lymph, a clear
fluid that bathes the body's tissues. Lymph, along with the many cells and
particles it carries-notably lymphocytes, macrophages, and foreign antigens,
drains out of tissues and seeps across the thin walls of tiny lymphatic vessels.
The vessels transport the mix to lymph nodes, where antigens can be filtered out
and presented to immune cells.
Additional lymphocytes reach the lymph
nodes (and other immune tissues) through the bloodstream. Each node is supplied
by an artery and a vein; lymphocytes enter the node by traversing the walls of
the very small specialized veins.
All lymphocytes exit lymph nodes in lymph
via outgoing lymphatic vessels. Much as small creeks and streams empty into
larger rivers, the lymphatics feed into larger and larger channels. At the base
of the neck, large lymphatic vessels merge into the thoracic duct, which empties
its contents into the bloodstream.
Once in the bloodstream, the lymphocytes
and other assorted immune cells are transported to tissues throughout the body.
They patrol everywhere for foreign antigens, then gradually drift back into the
lymphatic vessels, to begin the cycle all over again.
This information is a
collaboration of the National Cancer Institute
and the National Institute of Allergy and Infectious Diseases.
National Institutes of Health
National Cancer Institute
U.S. Department of Health and Human Services
Public Health Service
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