Page 829 - Fundamentals of anatomy physiology

816 Unit 4 Fluids and Transport
The T cells and B cells that migrate from their sites of origin
retain the ability to divide, producing daughter cells of the same
type. For example, a dividing B cell produces other B cells, not
T cells or NK cells. As we will see, the ability of specific types of organs—the lymph nodes, the thymus, and the spleen—from
lymphocytes to increase in number is crucial to the success of
the immune response.
Lymphoid Tissues
Lymphoid tissues are connective tissues dominated by lym-
phocytes. In a lymphoid nodule, or lymphatic nodule, the
lymphocytes are densely packed in an area of areolar tis-
sue. In many areas, lymphoid nodules form large clusters
(Figure 22–7). Lymphoid nodules occur in the connective
tissue deep to the epithelia lining the respiratory tract, where
they are known as tonsils, and along the digestive, respiratory,
urinary, and reproductive tracts. They are also found within
more complex lymphoid organs, such as lymph nodes or the
spleen.
A single nodule averages about a millimeter in diameter.
Its boundaries are not distinct, because no fibrous capsule
surrounds it. Each nodule often has a central zone called a
germinal center, which contains dividing lymphocytes
(Figure 22–7a,b).
22 Tonsils
The tonsils are large lymphoid nodules in the walls of the
pharynx (Figure 22–7a). Most people have five tonsils. A single
pharyngeal tonsil, often called the adenoid, lies in the poste-
rior superior wall of the nasopharynx. Left and right palatine
tonsils are located at the posterior, inferior margin of the oral
cavity, along the boundary with the pharynx. A pair of lingual
tonsils lie deep to the mucous epithelium covering the base
(pharyngeal portion) of the tongue. Because of their location,
the lingual tonsils are usually not visible unless they become
infected and swollen. Tonsillitis is an inflammation of the
tonsils (especially the palatines) although the other tonsils may
also be affected. Tonsils reach their largest size by puberty and
then begin to atrophy.
MALT
The collection of lymphoid tissues that protect the epithelia
of the digestive, respiratory, urinary, and reproductive systems
is called the mucosa-associated lymphoid tissue (MALT).
Clusters of lymphoid nodules deep to the epithelial lining of
the intestine are known as aggregated lymphoid nodules, or
Peyer’s patches (Figure 22–7b). Other examples of MALT include
the appendix and the tonsils.
The appendix, or vermiform (“worm-shaped”) appendix, is
a blind pouch that originates near the junction between the
small and large intestines. Its walls contain a mass of fused
lymphoid nodules.
Lymphoid Organs
A fibrous connective tissue capsule separates lymphoid
s­ urrounding tissues.
Lymph Nodes
Lymph nodes are small lymphoid organs ranging in diameter
from 1 mm to 25 mm (to about 1 in.). The greatest number
of lymph nodes is located in the neck, armpits, and groin,
where they defend us against bacteria and other invaders.
Figure 22–1 shows the general pattern of lymph node distribu-
tion in the body.
A dense connective tissue capsule covers each lymph node
(Figure 22–8). Bundles of collagen fibers extend from the cap-
sule into the interior of the node. These fibrous partitions are
called trabeculae (trabecula, a beam).
The typical lymph node is shaped like a kidney bean
(Figure 22–8). Blood vessels and nerves reach the lymph node
at a shallow indentation called the hilum. Two sets of lym-
phatic vessels, afferent lymphatics and efferent lymphatics, are
connected to each lymph node. Afferent (afferens, to bring to)
lymphatics bring lymph to the lymph node from peripheral
tissues. The afferent lymphatics penetrate the capsule of the
lymph node on the side opposite the hilum. Efferent (efferens,
to bring out) lymphatics leave the lymph node at the hilum.
These vessels carry lymph away from the lymph node and to-
ward the venous circulation.
Lymph Flow. Lymph from the afferent lymphatics flows
through the lymph node within a network of sinuses, open
passageways with incomplete walls (Figure 22–8). Lymph first
enters a subcapsular space (formerly called the subcapsular sinus).
It contains a meshwork of branching reticular fibers, macro-
phages, and dendritic cells. Dendritic cells are involved in
starting an immune response. (We consider their role in a later
section.) Lymph passes through the subcapsular space and then
flows through the outer cortex of the node. The outer cortex
contains B cells within germinal centers similar to those of
lymphoid nodules.
Lymph then continues through lymph sinuses in the deep
cortex (paracortical area), which is dominated by T cells. Here
lymphocytes leave the bloodstream and enter the lymph node
by crossing the walls of blood vessels.
After flowing through the sinuses of the deep cortex, lymph
continues into the core, or medulla, of the lymph node. The
medulla contains B cells and plasma cells organized into elon-
gate masses known as medullary cords. Lymph passes through
a network of sinuses in the medulla and then enters the efferent
lymphatics at the hilum.
Lymph Node Function. A lymph node functions like a kitchen
water filter. It purifies lymph before it reaches the veins. As