Chapter 20
The Lymphatic System and Lymphoid Organs and Tissues
is a tubular offshoot of the first part of the large
intestine and contains a high concentration of lymphoid follicles.
Like Peyer’s patches, the appendix is in an ideal position (1) to de-
stroy bacteria (which are present in large numbers in the intestine)
before these pathogens can breach the intestinal wall, and (2) to
generate many “memory” lymphocytes for long-term immunity.
Developmental Aspects of
the Lymphatic System and
Lymphoid Organs and Tissues
Outline the development of the lymphatic system and the
lymphoid organs and tissues.
By the fiFh week of embryonic development, the beginnings of
the lymphatic vessels and the main clusters of lymph nodes are ap-
parent. Tese arise as
lymph sacs
that bud from developing veins.
a maturation site for ± lymphocyte precursors. Tese precur-
sors must be kept isolated from foreign antigens to prevent
their premature activation. In fact, there is a
blood thymus
that keeps bloodborne antigens out of the thymus.
Te stroma of the thymus consists of epithelial cells rather
than reticular fibers. Tese epithelial cells provide the physical
and chemical environment in which ± lymphocytes mature.
Mucosa-Associated Lymphoid Tissue
Mucosa-associated lymphoid tissues (MALT)
are a set of distrib-
uted lymphoid tissues strategically located in mucous membranes
throughout the body (see ²igure 4.11 on p. 141 to review mucous
membranes). MAL± helps protect us from the never-ending on-
slaught of pathogens that seek to enter our bodies. Here we will
consider the largest collections of MAL±—the tonsils, Peyer’s
patches, and appendix. In addition to these large named collec-
tions, MAL± also occurs in the mucosa of the respiratory and
genitourinary organs as well as the rest of the digestive tract.
form a ring of lymphoid tissue around the entrance
to the pharynx (throat), where they appear as swellings of the
mucosa (
Figure 20.8
and ²igure 22.3). Te tonsils are named
according to location.
Te paired
palatine tonsils
are located on either side at the
posterior end of the oral cavity. Tese are the largest tonsils
and the ones most oFen infected.
lingual tonsil
is the collective term for a lumpy collec-
tion of lymphoid follicles at the base of the tongue.
pharyngeal tonsil
(referred to as the
if en-
larged) is in the posterior wall of the nasopharynx.
Te tiny
tubal tonsils
surround the openings of the auditory
tubes into the pharynx.
Te tonsils gather and remove many of the pathogens entering
the pharynx in food or in inhaled air.
Te lymphoid tissue of the tonsils contains follicles with
obvious germinal centers surrounded by diffusely scattered
lymphocytes. Te tonsils are not fully encapsulated, and the epi-
thelium overlying them invaginates deep into their interior, form-
ing blind-ended
tonsillar crypts
(²igure 20.8). Te crypts trap
bacteria and particulate matter, and the bacteria work their way
through the mucosal epithelium into the lymphoid tissue, where
most are destroyed. It seems a bit dangerous to “invite” infection
this way, but this strategy produces a wide variety of immune cells
that have a “memory” for the trapped pathogens. In other words,
the body takes a calculated risk early on (during childhood) for
the benefits of heightened immunity and better health later.
Peyer’s Patches
Peyer’s patches
erz), or
aggregated lymphoid nodules
, are
large clusters of lymphoid follicles, structurally similar to the
tonsils. Tey are located in the wall of the distal portion of the
small intestine (²igure 20.5 and
Figure 20.9
Germinal centers
in lymphoid follicles
Pharyngeal tonsil
Palatine tonsil
Lingual tonsil
Figure 20.8
Histology of the palatine tonsil.
The exterior
surface of the tonsil is covered by stratified squamous epithelium,
which invaginates deeply to form tonsillar crypts (10
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