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[ Pobierz całość w formacie PDF ]22
The Lymphoid System
and Immunity
Did you know...?
The greatest number of lymph nodes is located
in the neck, armpits, and groin where they
defend us against bacteria and other invaders.
Learning Outcomes
After completing this chapter, you should be able to do the following:
22-1
Distinguish between nonspecific and specific defense, and
explain the role of lymphocytes in the immune response.
22-2
Identify the major components of the lymphoid system,
describe the structure and functions of each component, and
discuss the importance of lymphocytes.
22-3
List the body’s nonspecific defenses, and describe the
components, mechanisms, and functions of each.
22-4
Define specific defenses, identify the forms and properties of
immunity, and distinguish between cell-mediated (cellular)
immunity and antibody-mediated (humoral) immunity.
22-5
Discuss the types of T cells and their roles in the immune
response, and describe the mechanisms of T cell activation and
differentiation.
22-6
Discuss the mechanisms of B cell activation and differentiation,
describe the structure and function of antibodies, and explain
the primary and secondary responses to antigen exposure.
22-7
Describe the development of immunological competence, list
and explain examples of immune disorders and allergies, and
discuss the effects of stress on immune function.
22-8
Describe the effects of aging on the lymphoid system and the
immune response.
22-9
Give examples of interactions between the lymphoid system
and each of the other organ systems.
Clinical Notes
Cancer and the Lymphoid System p. 786
Graft Rejection and Immunosuppression p. 803
AIDS p. 819
777
Chapter 22
The Lymphoid System and Immunity
An Introduction to the Lymphoid
System and Immunity
mechanisms are called
nonspecific defenses
, because they do not
distinguish one potential threat from another. In contrast, lym-
phocytes respond specifically: If a bacterial pathogen invades pe-
ripheral tissues, lymphocytes organize a defense against that
particular type of bacterium. For this reason, lymphocytes are
said to provide a
specific defense
, known as the
immune re-
sponse
. The ability to resist infection and disease through the
activation of specific defenses constitutes
immunity
.
All the cells and tissues involved in the production of im-
munity are sometimes considered part of an
immune system
—
a physiological system that includes not only the lymphoid
system, but also components of the integumentary, cardiovas-
cular, respiratory, digestive, and other systems. For example,
interactions between lymphocytes and dendritic (Langer-
hans) cells of the skin are important in mobilizing specific de-
fenses against skin infections.
We begin this chapter by examining the organization of
the lymphoid system. We will then consider the body’s non-
specific defenses. Finally, we will see how the lymphoid sys-
tem interacts with cells and tissues of other systems to defend
the body against infection and disease.
Many organs and systems work together to keep us alive and
healthy. In the ongoing struggle to maintain health, the lym-
phoid system plays a central role. This chapter discusses the
components of the lymphoid system and the ways those com-
ponents interact.
22-1
Anatomical barriers and
defense mechanisms constitute
nonspecific defense, and
lymphocytes provide specific
defense
The world is not always kind to the human body. Accidental
bumps, cuts, and scrapes; chemical and thermal burns; ex-
treme cold; and ultraviolet radiation are just a few of the haz-
ards in our physical environment. Making matters worse, the
world around us contains an assortment of viruses, bacteria,
fungi, and parasites capable of not only surviving but thriving
inside our bodies—and potentially causing us great harm.
These organisms, called
pathogens,
are responsible for many
diseases in humans. Each pathogen has a different lifestyle
and attacks the body in a specific way. For example, viruses
spend most of their time hidden within cells, which they of-
ten eventually destroy, whereas some of the largest parasites
actually burrow through internal organs. Many bacteria multi-
ply in interstitial fluids, where they release foreign proteins—
enzymes or toxins—that can damage cells, tissues, even entire
organ systems. And as if that were not enough, we are con-
stantly at risk from renegade body cells that have the poten-
tial to produce lethal cancers.
l
p. 105
The
lymphoid system
includes the cells, tissues, and or-
gans responsible for defending the body against both environ-
mental hazards, such as various pathogens, and internal
threats, such as cancer cells.
Lymphocytes
, the primary cells of
the lymphoid system, were introduced in Chapters 4 and 19.
l
pp. 131, 668
These cells are vital to the body’s ability to
resist or overcome infection and disease. Lymphocytes re-
spond to the presence of invading pathogens (such as bacter-
ia or viruses), abnormal body cells (such as virus-infected
cells or cancer cells), and foreign proteins (such as the toxins
released by some bacteria). They act to eliminate these threats
or render them harmless through a combination of physical
and chemical attacks.
The body has several anatomical barriers and defense mech-
anisms that either prevent or slow the entry of infectious organ-
isms, or attack them if they do succeed in gaining entry. These
CHECKPOINT
1. Define pathogen.
2. Explain the difference between nonspecific defense
and specific defense.
See the blue Answers tab at the end of the book.
22-2
Lymphatic vessels,
lymphocytes, lymphoid tissues,
and lymphoid organs function
in body defenses
The lymphoid system consists of (1)
lymph,
a fluid that re-
sembles plasma but contains a much lower concentration of
suspended proteins; (2) a network of
lymphatic vessels,
of-
ten called
lymphatics,
which begin in peripheral tissues and
end at connections to veins; (3) an array of
lymphoid tissues
and
lymphoid organs
scattered throughout the body; and (4)
lymphocytes and smaller numbers of phagocytes and other
cells.
Figure 22–1
provides a general overview of the primary
tissues, vessels, and organs of this system.
Functions of the Lymphoid System
The primary function of the lymphoid system is the production,
maintenance, and distribution of lymphocytes that provide de-
778
Unit 4
Fluids and Transport
To provide an effective defense, lymphocytes must be
able to detect problems, and they must be able to reach the
site of injury or infection. Lymphocytes, macrophages, and
microphages circulate within the blood and are able to enter
or leave the capillaries that supply most of the tissues of the
body. As noted in Chapter 21, capillaries normally deliver
more fluid to peripheral tissues than they carry away.
l
p. 736
The excess fluid returns to the bloodstream through
lymphatic vessels. This continuous circulation of extracellu-
lar fluid helps transport lymphocytes and other defense cells
from one organ to another. In the process, it maintains nor-
mal blood volume and eliminates local variations in the com-
position of the interstitial fluid by distributing hormones,
nutrients, and waste products from their tissues of origin to
the general circulation.
Lymphatics
of upper limb
Tonsil
Cervical
lymph nodes
Right
lymphatic
duct
Thymus
Axillary
lymph nodes
Thoracic
duct
Thoracic
duct
Lymphatics of
mammary gland
Spleen
Cisterna
chyli
Lumbar
lymph
nodes
Mucosa-associated
lymphoid tissue
(MALT) in
intestinal tract
Lymphatic Vessels
Lymphatic vessels carry lymph from peripheral tissues to the
venous system. The smallest lymphatic vessels are called
lymphatic capillaries
.
Pelvic
lymph nodes
Inguinal
lymph nodes
Lymphatic Capillaries
The lymphatic network begins with
lymphatic capillaries,
or
terminal lymphatics
, which branch through peripheral tis-
sues. Lymphatic capillaries differ from blood capillaries in
that they (1) originate as pockets rather than forming contin-
uous tubes, (2) have larger diameters, (3) have thinner walls,
and (4) typically have a flattened or irregular outline in sec-
tional view (
Figure 22–2
). Although lymphatic capillaries are
lined by endothelial cells, the basal lamina is incomplete or
absent. The endothelial cells of a lymphatic capillary are not
bound tightly together, but they do overlap. The region of
overlap acts as a one-way valve, permitting the entry of fluids
and solutes (including those as large as proteins), as well as
viruses, bacteria, and cell debris, but preventing their return
to the intercellular spaces.
Lymphatic capillaries are present in almost every tissue
and organ in the body. Prominent lymphatic capillaries in the
small intestine called
lacteals
are important in the transport of
lipids absorbed by the digestive tract. Lymphatic capillaries
are absent in areas that lack a blood supply, such as the cornea
of the eye. The bone marrow and the central nervous system
also lack lymphatic vessels.
Appendix
Lymphatics
of lower
limb
Figure 22–1
An Overview of the Lymphoid System: The
Lymphatic Vessels, Lymphoid Tissues, and Lymphoid Organs.
Small Lymphatic Vessels
From the lymphatic capillaries, lymph flows into larger lym-
phatic vessels that lead toward the body’s trunk. The walls of
these vessels contain layers comparable to those of veins, and,
like veins, the larger lymphatic vessels contain valves (
Figure
22–3
). The valves are quite close together, and at each the
fense against infections and other environmental hazards. Most
of the body’s lymphocytes are produced and stored within lym-
phoid tissues (such as the tonsils) and lymphoid organs (such
as the spleen and thymus). However, lymphocytes are also pro-
duced in areas of red bone marrow, along with other defense
cells, such as monocytes and macrophages.
779
Chapter 22
The Lymphoid System and Immunity
lymphatic vessel bulges noticeably. As a result, large lym-
phatic vessels have a beaded appearance (
Figure 22–3a
). The
valves prevent the backflow of lymph within lymph vessels,
especially those of the limbs. Pressures within the lymphoid
system are minimal, and the valves are essential to maintain-
ing normal lymph flow toward the thoracic cavity.
Lymphatic vessels commonly occur in association with
blood vessels (
Figure 22–3a
). Differences in relative size, gen-
eral appearance, and branching pattern distinguish lymphatic
vessels from arteries and veins. Characteristic color differ-
ences are also apparent on examining living tissues. Most ar-
teries are bright red, veins are dark red (although usually
illustrated as blue to distinguish them from arteries), and
lymphatic vessels are a pale golden color. In general, a tissue
contains many more lymphatic vessels than veins, but the
lymphatic vessels are much smaller.
Smooth
muscle
Arteriole
Endothelial
cells
Lymphatic
capillary
Major Lymph-Collecting Vessels
Two sets of lymphatic vessels collect lymph from the lym-
phatic capillaries: superficial lymphatics and deep lymphatics.
Superficial lymphatics
are located in the subcutaneous layer
deep to the skin; in the areolar tissues of the mucous mem-
branes lining the digestive, respiratory, urinary, and reproduc-
tive tracts; and in the areolar tissues of the serous membranes
lining the pleural, pericardial, and peritoneal cavities.
Deep
lymphatics
are larger lymphatic vessels that accompany deep
arteries and veins supplying skeletal muscles and other organs
of the neck, limbs, and trunk, and the walls of visceral organs.
Superficial and deep lymphatics converge to form even
larger vessels called
lymphatic trunks
, which in turn empty
into two large collecting vessels: the thoracic duct and the
right lymphatic duct. The
thoracic duct
collects lymph from
the body inferior to the diaphragm and from the left side of
the body superior to the diaphragm. The smaller
right lym-
phatic duct
collects lymph from the right side of the body su-
perior to the diaphragm (
Figure 22–4a
).
The
thoracic duct
begins inferior to the diaphragm at the
level of vertebra L
2
(
Figure 22–4b
). The base of the thoracic
duct is an expanded, saclike chamber called the
cisterna
chyli
(KI-1i;
chylos
, juice). The cisterna chyli receives lymph
from the inferior part of the abdomen, the pelvis, and the
lower limbs by way of the
right
and
left lumbar trunks
and the
intestinal trunk
.
The inferior segment of the thoracic duct lies anterior to
the vertebral column. From the second lumbar vertebra, it
passes posterior to the diaphragm alongside the aorta and as-
cends along the left side of the vertebral column to the level
of the left clavicle. After collecting lymph from the
left bron-
chomediastinal trunk
, the
left subclavian trunk
, and the
left
jugular trunk
, it empties into the left subclavian vein near the
left internal jugular vein (
Figure 22–4b
). Lymph collected
Venule
Interstitial
Interstitial
fluid
Interstitial
fluid
fluid
Lymph
Lymph
flow
Lymph
flow
flow
Blood capillaries
Loose connective tissue
(a) Association of blood capillaries, tissue, and lymphatic capillaries
Lymphocyte
Lymph
Lymph
flow
Lymph
flow
Incomplete
Incomplete
basal
Incomplete
basal
lamina
flow
basal
lamina
lamina
To larger
To larger
lymphatics
To larger
lymphatics
Loose
connective
Loose
connective
tissue
Loose
lymphatics
connective
tissue
tissue
Interstitial fluid
Interstitial fluid
Interstitial
Interstitial
fluid
Interstitial
fluid
Lymphatic
Lymphatic
capillary
Lymphatic
capillary
fluid
capillary
Blood
capillary
Blood
capillary
Blood
capillary
(b) Sectional view
Figure 22–2
Lymphatic Capillaries.
(a)
The interwoven network
formed by blood capillaries and lymphatic capillaries. Arrows
indicate the movement of fluid out of blood vessels and the net flow
of interstitial fluid and lymph.
(b)
A sectional view indicating the
movement of fluid from the plasma, through the interstitial fluid,
and into the lymphoid system.
780
Unit 4
Fluids and Transport
Artery
Vein
Artery
Vein
Lymphatic vessel
Lymphatic
vessel
Toward
venous
system
Lymphatic valve
(a)
From lymphatic
capillaries
Lymphatic valve
Lymphatic vessel
(b)
LM
63
Figure 22–3
Lymphatic Vessels and Valves. (a)
A diagrammatic view of loose connective tissue containing small blood vessels and a
lymphatic vessel. The cross-sectional view emphasizes the structural differences among these structures.
(b)
A lymphatic valve. Like valves in
veins, each lymphatic valve consists of a pair of flaps that permit movement of fluid in only one direction.
from the left side of the head, neck, and thorax, as well as
lymph from the entire body inferior to the diaphragm, reen-
ters the venous circulation in this way.
The
right lymphatic duct
is formed by the merging of
the
right jugular
,
right subclavian
, and
right bronchomediasti-
nal trunks
in the area near the right clavicle. This duct emp-
ties into the right subclavian vein, delivering lymph from the
right side of the body superior to the diaphragm.
Blockage of the lymphatic drainage from a limb produces
lymphedema
(limf-e-DE-muh), a condition in which inter-
stitial fluids accumulate and the limb gradually becomes
swollen and grossly distended. If the condition persists, the
connective tissues lose their elasticity and the swelling be-
comes permanent. Lymphedema by itself does not pose a ma-
jor threat to life. The danger comes from the constant risk that
an uncontrolled infection will develop in the affected area. Be-
cause the interstitial fluids are essentially stagnant, toxins and
pathogens can accumulate and overwhelm local defenses
without fully activating the immune system.
Lymphocytes
Lymphocytes
account for 20–30 percent of the circulating
leukocyte population. However, circulating lymphocytes are
only a small fraction of the total lymphocyte population. The
body contains some 10
12
lymphocytes, with a combined
weight of more than a kilogram (2.2 lbs).
Types of Lymphocytes
Three classes of lymphocytes circulate in blood: (1)
T
(
t
hymus-
dependent)
cells,
(2)
B
(
b
one marrow–derived)
cells,
and
(3)
NK
(
n
atural
k
iller)
cells
. Each type has distinctive biochem-
ical and functional characteristics.
Approximately 80 percent of circulating lymphocytes
are classified as T cells. The primary types of T cells are the
following:
•
Cytotoxic T cells,
which attack foreign cells or body
cells infected by viruses. Their attack commonly involves
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