656
Chapter 18
(a)
(b)
Afferent lymphatic
vessels
Lymphatic
(lymphocyte)
nodules
Valve
Efferent lymphatic
vessel
Sinuses
There are no anatomical links, other than via the car-
diovascular system, between the various lymphoid organs. Let
us look briefl y at these organs, with the exception of the bone
marrow, which was described in Chapter 12.
The
thymus
lies in the upper part of the chest. Its size
varies with age, being relatively large at birth and continu-
ing to grow until puberty, when it gradually atrophies and is
replaced by fatty tissue. Before its atrophy, the thymus consists
mainly of mature lymphocytes that will eventually migrate via
the blood to the secondary lymphoid organs. It also contains
endocrine cells that secrete a group of hormones, collectively
called
thymopoietin,
that exert a still poorly understood reg-
ulatory effect on lymphocytes of thymic origin.
Recall from Chapter 12 that the fl uid fl owing along the
lymphatic vessels is called lymph, which is interstitial fl uid that
has entered the lymphatic capillaries and is routed to the large
lymphatic vessels that drain into systemic veins. During this
trip, the lymph fl ows through
lymph nodes
scattered along
the vessels. Lymph, therefore, is the route by which lympho-
cytes in the lymph nodes encounter the antigens that acti-
vate them. Each node is a honeycomb of lymph-fi lled sinuses
(
Figure 18–7
) with large clusters of lymphocytes (the lym-
phatic nodules) between the sinuses. They also contain many
macrophages and dendritic cells.
Figure 18–7
Anatomy of a lymph node as seen in (a) a sketch and in (b) a section viewed by light-microscopy.
Figure 18–7
physiological
inquiry
The nonspecifi c immune response includes vasodilation of the microcirculation and an increase in protein permeability of the capillaries (see
Table 18–3). How might these changes enhance the specifi c immune response during an infection?
Answer can be found at end of chapter.
The
spleen
is the largest of the secondary lymphoid
organs and lies in the left part of the abdominal cavity between
the stomach and the diaphragm. The spleen is to the circu-
lating blood what the lymph nodes are to the lymph. Blood
percolates through the vascular meshwork of the spleen’s inte-
rior, where large collections of lymphocytes, macrophages,
and dendritic cells are found. The macrophages of the spleen,
in addition to interacting with lymphocytes, also phagocytize
aging or dead erythrocytes.
The
tonsils
and
adenoids
are a group of small, rounded
lymphoid organs in the pharynx. They are fi
lled with lym-
phocytes, macrophages, and dendritic cells, and they have
openings (“crypts”) to the surface of the pharynx. Their lym-
phocytes respond to microbes that arrive by way of ingested
food as well as through inspired air. Similarly, the lympho-
cytes in the linings of the various tracts exposed to the exter-
nal environment respond to infectious agents that penetrate
the linings from the lumen of the tract.
At any moment in time, some lymphocytes are on their
way from the bone marrow or thymus to the secondary lym-
phoid organs. The vast majority, though, are cells that are
participating in lymphocyte traffi c
between
the secondary lym-
phoid organs, blood, lymph, and all the tissues of the body.
Lymphocytes from all the secondary lymphoid organs con-
stantly enter the lymphatic vessels that drain them (all lymphoid
organs, not just lymph nodes, are drained by lymphatic vessels)
and are carried to the blood. Simultaneously, some blood lym-
phocytes are pushing through the endothelium of venules all
over the body to enter the interstitial fl
uid. From there, they
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