Chapter 20
The Lymphatic System and Lymphoid Organs and Tissues
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20
usually embedded in connective tissue, they are not ordinarily
visible. Large clusters of lymph nodes occur near the body sur-
face in the inguinal, axillary, and cervical regions, places where
the collecting lymphatic vessels converge to form trunks (see
Figure 20.2a).
Lymph nodes have two basic functions, both concerned with
body protection:
Filtration.
As lymph is transported back to the bloodstream,
the lymph nodes act as lymph “filters.” Macrophages in the
nodes remove and destroy microorganisms and other debris
that enter the lymph from the loose connective tissues, pre-
venting them from being delivered to the blood and spread-
ing to other parts of the body.
Immune system activation.
Lymph nodes and other lym-
phoid organs are strategically located sites where lym-
phocytes encounter antigens and are activated to mount
an attack against them. Let’s look at how the structure of a
lymph node supports its defensive functions.
Structure of a Lymph Node
Lymph nodes vary in shape and size, but most are bean shaped
and less than 2.5 cm (1 inch) in length. Each node is surrounded
by a dense fibrous
capsule
from which connective tissue strands
called
trabeculae
extend inward to divide the node into a
number of compartments
(Figure 20.4)
. Te node’s internal
framework, or stroma, of reticular fibers physically supports its
ever-changing population of lymphocytes.
A lymph node has two histologically distinct regions, the
cor-
tex
and the
medulla
. Te superficial part of the cortex contains
densely packed follicles, many with germinal centers heavy with
dividing B cells. Dendritic cells nearly encapsulate the follicles and
abut the deeper part of the cortex, which primarily houses ± cells
in transit. Te ± cells circulate continuously between the blood,
lymph nodes, and lymph, performing their surveillance role.
Medullary cords
are thin inward extensions from the cor-
tical lymphoid tissue, and contain both types of lymphocytes.
Troughout the node are
lymph sinuses
, large lymph capillar-
ies spanned by crisscrossing reticular fibers. Numerous macro-
phages reside on these reticular fibers and phagocytize foreign
matter in the lymph as it flows by in the sinuses. Additionally,
some of the lymph-borne antigens in the percolating lymph
leak into the surrounding lymphoid tissue, where they activate
lymphocytes to mount an immune attack against them.
Circulation in the Lymph Nodes
Lymph enters the convex side of a lymph node through a
number of
afferent
lymphatic vessels
. It then moves through
a large, baglike sinus, the
subcapsular sinus
, into a number of
smaller sinuses that cut through the cortex and enter the me-
dulla. Te lymph meanders through these
medullary sinuses
and finally exits the node at its
hilum
(hi
9
lum), the indented
region on the concave side, via
efferent
lymphatic vessels
.
Tere are fewer efferent vessels draining the node than af-
ferent vessels feeding it, so the flow of lymph through the node
venules coursing through this network and temporarily occupy
its spaces (Figure 20.3). Ten, they leave to patrol the body again.
Te cycling of lymphocytes between the circulatory vessels, lym-
phoid tissues, and loose connective tissues of the body ensures
that lymphocytes reach infected or damaged sites quickly.
Lymphoid tissue comes in various “packages,” such as diffuse
lymphoid tissue and lymphoid follicles.
Diffuse lymphoid tissue
—a loose arrangement of lymphoid
cells and some reticular fibers—is found in virtually every
body organ. Larger collections appear in the lamina propria of
mucous membranes such as those lining the digestive tract.
Lymphoid follicles (lymphoid nodules)
are solid, spheri-
cal bodies consisting of tightly packed lymphoid cells and
reticular fibers. Follicles o²en have lighter-staining
germinal
centers
where proliferating B cells predominate. Tese cen-
ters enlarge dramatically when the B cells are dividing rap-
idly and producing plasma cells. In many cases, the follicles
form part of larger lymphoid organs, such as lymph nodes.
However, isolated aggregations of lymphoid follicles occur
in the intestinal wall as Peyer’s patches (aggregated lymphoid
nodules) and in the appendix (see p. 759).
Lymph Nodes
Describe the general location, histological structure, and
functions of lymph nodes.
Te principal lymphoid organs in the body are the
lymph
nodes
, which cluster along the lymphatic vessels of the body.
Tere are hundreds of these small organs, but because they are
Macrophage
Medullary sinus
Reticular fiber
Lymphocytes
Reticular cells on
reticular fibers
Figure 20.3
Reticular connective tissue in a human lymph
node.
Scanning electron micrograph (690
3
).
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