623
CHAPTER SIXTEEN
Lymphatic System and Immunity
the bloodstream, and monitoring body fluids (immune
surveillance) provided by lymphocytes and macrophages.
Along with the red bone marrow, the lymph nodes are cen-
ters for lymphocyte production. These cells attack viruses,
bacteria, and other parasitic cells that lymphatic vessels
bring to the lymph nodes. Macrophages in the lymph nodes
engulf and destroy foreign substances, damaged cells, and
cellular debris.
PRACTICE
9
Distinguish between a lymph node and a lymph nodule.
10
What factors promote the F
ow of lymph through a node?
11
In what body regions are lymph nodes most abundant?
12
What are the major functions of lymph nodes?
16.6
THYMUS AND SPLEEN
Two other lymphatic organs, whose functions are similar to
those of the lymph nodes, are the thymus and the spleen.
Thymus
The
thymus
(thi
mus) is a soft, bilobed structure enclosed in
a connective tissue capsule. It is in the mediastinum, anterior
to the aortic arch and posterior to the upper part of the body
of the sternum, and extends from the root of the neck to the
pericardium
(f g. 16.12
a
)
. The thymus varies in size and is
usually proportionately larger during infancy and early child-
hood. After puberty, the thymus shrinks, and in an adult, it
may be small
(f g. 16.13)
. In elderly persons, adipose and
connective tissues replace lymphatic tissue in the thymus.
Connective tissues extend inward from the surface of
the thymus, subdividing it into lobules (see F
g. 16.12
b
).
The lobules house many lymphocytes that developed from
progenitor cells in the bone marrow. Most of these cells
(thymocytes) are inactivated; however, some mature into
T lymphocytes,
or (T cells)which leave the thymus and
provide immunity. Epithelial cells in the thymus secrete
protein hormones called
thymosins,
which stimulate matu-
ration of T lymphocytes.
Spleen
The
spleen
(sple
¯n) is the largest lymphatic organ. It is in the
upper left portion of the abdominal cavity, just inferior to
the diaphragm, posterior and lateral to the stomach (see F g.
16.12
a
and reference plates 4, 5, and 6).
The spleen resembles a large lymph node in that it is
enclosed in connective tissue that extends inward from the
surface and partially subdivides the organ into chambers, or
lobules. The organ also has a hilum on one surface through
which blood vessels and nerves enter. However, unlike
the sinuses of a lymph node that are F lled with lymph, the
spaces (venous sinuses) in the chambers of the spleen are
F
lled with blood.
6.
Abdominal cavity.
These lymph nodes form chains
along the main branches of the mesenteric arteries and
the abdominal aorta. These lymph nodes receive lymph
from the abdominal viscera.
7.
Thoracic cavity.
These lymph nodes are in the
mediastinum and along the trachea and bronchi. They
receive lymph from the thoracic viscera and from the
internal wall of the thorax.
The illness described as “swollen glands” refers to enlarged cervical
lymph nodes associated with throat or respiratory infection.
Functions of Lymph Nodes
Lymph nodes have two primary functions: F
ltering poten-
tially harmful particles from lymph before returning it to
FIGURE 16.11
Major locations of lymph nodes.
Thoracic
lymph
node
Cervical
lymph
node
Axillary
lymph
node
Supratrochlear
lymph
node
Abdominal
lymph
node
Pelvic
lymph
node
Inguinal
lymph
node
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