Page 165 - 84 human physiolofy part-1

The Immune System

thymus. Here they mature and for the most part, stay in the thymus. Only 5% of T lymphocytes ever
leave the thymus. They only leave if they are able to pass the test: if they react with “self” cells, they
die. If they have the potential to attack a foreign cell, they leave the thymus.

Secondary Lymphatic OrgansThe secondary lymphatic organs also play an important role in the
immune system as they are places where lymphocytes find and bind with antigens This is followed by
the proliferation and activation of lymphocytes. The secondary organs include the spleen, lymph nodes,
tonsils, Preyer’s patches, and the appendix.

• The spleen, The spleen is a ductless, vertebrate gland that is closely associated with the
circulatory system, where it functions in the destruction of old red blood cells in holding a
reservoir of blood.located in the upper left region of the abdominal cavity, is divided into partial
compartments. Each compartment contains tissue known as white pulp and red pulp. The white
pulp contains lymphocytes and the red pulp acts in blood filtration. When blood enters the
spleen and flows through the sinuses for filtration, lymphocytes react to pathogens ,
macrophages engulf debris, and also remove old, worn out red blood cells. A person without a
spleen is more susceptible to infections and may need supplementary antibiotic therapy for the
rest of their life.

• Lymph Nodes are small oval shaped structures located along the lymphatic vessels. They
are about 1-25 mm in diameter. Lymph nodes act as filters, with an internal honeycomb of
connective tissue filled with lymphocytes that collect and destroy bacteria and viruses. They are
divided into compartments, each packed with B lymphocytes and a sinus. As lymph flows
through the sinuses, it is filtered by macrophages whose function is to engulf pathogens and
debris. Also present in the sinuses are T lymphocytes, whose functions are to fight infections
and attack cancer cells. Lymph nodes are in each cavity of the body except the dorsal cavity.
Physicians can often detect the body’s reaction to infection by feeling for swollen, tender lymph
nodes under the arm pits and in the neck, because when the body is fighting an infection, these
lymphocytes multiply rapidly and produce a characteristic swelling of the lymph nodes.

• Tonsils are often the first organs to encounter pathogens and antigens that come into the
body by mouth or nose. There are 3 pairs of tonsils in a ring about the pharynx.

• Peyer’s patches, located in the wall of the intestine and the appendix, attached to the
cecum of the large intestine, intercept pathogens that come into the body through the intestinal
tract.

Leukocytes

The primary cells of the immune system are the leukocytes or white blood cells(WBC). Most
leukocytes are much larger then red blood cells, but they are not nearly as numerous. A microliter of
whole blood contains about 5 million red blood cells but only about 7000 leukocytes.

Although most leukocytes circulate through the blood, they usually leave the capillaries and
function extravascularly (outside the vessels). Some types of leukocytes can live out in the tissue for
several months, but others may live for only hours or days. Leukocytes can be distinguished from one
another in stained tissue samples by the shape and size of the nucleus, the staining characterisitcs of the
cytoplasm and the cytoplasmic inclusions, and the regularity of the cell border.

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