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Chapter 21: Immune System
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INTRODUCTION
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The immune system protects against
assaults on the body
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External assaults include microorganisms:
protozoans, bacteria, and viruses
Internal assaults: abnormal cells reproduce
and form tumors that may become cancerous
and spread
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ORGANIZATION OF THE
IMMUNE SYSTEM
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The immune system is continually at work
patrolling and protecting the body
Identification of cells and other particles
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Markers or antigens are unique molecules recognized by
the immune system
Self-markers: molecules on the surface of cells that are
unique to an individual, thus identifying the cell as “self”
to the immune system
Non–self-markers: molecules on the surface of foreign or
abnormal cells or particles that identify the particle as
“non-self” to the immune system
Self-tolerance: the ability of the immune system to attack
abnormal or foreign cells but spare normal cells
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ORGANIZATION OF THE
IMMUNE SYSTEM (cont.)
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Two major categories of immune mechanisms:
innate immunity and adaptive immunity (Figure
21-1; Table 21-1)
Innate immunity provides a general, nonspecific
defense against anything that is not “self”
Adaptive immunity acts as a specific defense
against specific threatening agents
Primary cells for innate immunity: epithelial barrier
cells, phagocytes (neutrophils, macrophages,
dendritic cells), and natural killer cells; chemicals
used in innate immunity: complement and
interferon
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ORGANIZATION OF THE
IMMUNE SYSTEM (cont.)
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Primary types of cells for adaptive
immunity: lymphocytes called T cells and B
cells
Cytokines: any of several kinds of
chemicals released by cells to promote
innate and adaptive immune responses
(e.g., interleukin, interferon, leukotriene)
Other chemicals (e.g., complement, other
enzymes, histamine) also play regulatory
roles in immunity
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INNATE IMMUNITY
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Species resistance: genetic characteristics
of an organism or species that defend
against pathogens (Table 21-2)
Mechanical and chemical barriers: first line
of defense (Figure 21-2)
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Internal environment of the body is protected
by a barrier composed of the skin and mucous
membranes
 Skin and mucous membranes provide
additional immune mechanisms: sebum,
mucus, enzymes, and hydrochloric acid in the
stomach
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INNATE IMMUNITY (cont.)
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Inflammation and fever: second line of
defense (Figure 21-3)
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Inflammatory response: tissue damage elicits
responses to counteract injury and promote
normalcy
• Inflammation mediators include histamine, kinins,
prostaglandins, and related compounds (Figure 21-4)
• Chemotactic factors: substances that attract white
blood cells to the area in a process called
chemotaxis
• Characteristic signs of inflammation: heat, redness,
pain, and swelling
• Systemic inflammation: occurs from a bodywide
inflammatory response
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INNATE IMMUNITY (cont.)
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Fever: abnormally high temperature triggered
by inflammation mediators
• Triggered in systemic inflammatory response
syndrome and events such as viral infections,
tumors, allergies
• Pyrogens released from damaged tissues
(endogenous) or introduced into the body
(exogenous) promote prostaglandin (PG) production;
PGs then reset the hypothalamic “thermostat” to a
higher temperature; aspirin and other
cyclooxygenase inhibitors interfere with PG
production
• Fever is believed to increase immune function and
inhibit pathogens
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INNATE IMMUNITY (cont.)
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Phagocytosis: ingestion and destruction of
microorganisms or other small particles by
phagocytes (Figure 21-7)
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Antigen-presenting cells: phagocytes that ingest foreign
particles, isolate protein segments (peptides), and
display them as antigens on their surface to trigger an
immune response when recognized by a specific
(adaptive) immune cell
Chemotaxis: chemical attraction of cells to the source of
the chemical attractant (Figure 21-6)
Diapedesis: process by which immune cells squeeze
through the wall of a blood vessel to get to the site of
injury or infection (Figure 21-5)
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INNATE IMMUNITY (cont.)
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Neutrophil: most numerous phagocyte; usually
first to arrive at site of injury; migrates out of
bloodstream during diapedesis; forms pus
Phagocytes (Table 21-3)
• Macrophage: large phagocytic cells
• Dendritic cell: type of phagocytic antigen-presenting
cell with long branches or extensions (Figure 21-8)
• Examples are histiocytes in connective tissue,
microglia in nervous system, and Kupffer cells in liver
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INNATE IMMUNITY (cont.)
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Natural killer (NK) cells: lymphocytes that kill
tumor cells and cells infected by viruses (Figure
21-9)
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Method of recognizing abnormal or non-self cells: target
cell is killed if killer-inhibiting receptor on NK cell does
not bind to a proper major histocompatibility complex
surface protein
Method of killing cells: lysing cells by damaging plasma
membranes
Interferon: protein synthesized and released into
circulation by certain cells if invaded by viruses to
signal other nearby cells to enter a protective
antiviral state
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INNATE IMMUNITY (cont.)
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Complement: group of enzymes that produce a
cascade of reactions resulting in a variety of
immune responses (Figure 21-10)
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Lyse cells when activated by either adaptive or innate
mechanisms
Opsonization: mark cells for destruction by phagocytes
Variety of other immune responses
Toll-like receptors: pattern recognition receptors in
the membranes of host cells; when triggered,
stimulate many different kinds of innate immune
responses
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OVERVIEW OF ADAPTIVE
IMMUNITY
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Adaptive immunity is part of the third line of
defense consisting of lymphocytes—two different
classes of a type of white blood cell (Figure 21-11)
Two classes of lymphocytes (Figure 21-12): B
lymphocytes (B cells) and T lymphocytes (T cells)
Subsets of lymphocytes are defined by the CD
surface markers that the cells carry (e.g., CD4
and CD8 cells)
Lymphocytes flow through the bloodstream,
become distributed in tissues, and return to the
bloodstream in a continuous recirculation
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OVERVIEW OF ADAPTIVE
IMMUNITY (cont.)
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B-cell mechanisms: antibody-mediated immunity
(humoral immunity); produce antibodies that attack
pathogens (Figure 21-13)
T cells attack pathogens more directly; classified as
cell-mediated immunity (cellular immunity)
Lymphocytes have protein markers on their surfaces
that are named by the CD system
Activation of lymphocytes requires two stimuli: a
specific antigen and activating chemicals (Figure 21-14)
Lymphocytes are densest where they develop—in bone
marrow, thymus gland, lymph nodes, and spleen
(Figure 21-15)
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B CELLS AND ANTIBODYMEDIATED IMMUNITY
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B cells develop in two stages
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Pre–B cells develop by a few months of age
The second stage occurs in lymph nodes and spleen—
activation of a naive B cell after it binds to a specific
antigen
B cells serve as ancestors to antibody-secreting plasma
cells
Antibodies: proteins (immunoglobulins) secreted
by activated B cell (Figure 21-16)
Structure of antibody molecules: an antibody
molecule consists of two heavy and two light
polypeptide chains; each molecule has two
antigen-binding sites and two complement-binding
sites (Figure 21-17)
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B CELLS AND ANTIBODYMEDIATED IMMUNITY (cont.)
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Diversity of antibodies: infants are born with different clones
of B cells in bone marrow, lymph nodes, and spleen; cells of
the clone synthesize a specific antibody with a sequence of
amino acids in its variable region that differs from the
sequence synthesized by other clones
Five classes of antibodies: immunoglobulins M, G, A, E, and
D (Figure 21-18)
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IgM: antibody that naive B cells synthesize and insert into their
own plasma membranes; the predominant class produced after
initial contact with an antigen
IgG: makes up 75% of antibodies in the blood; predominant
antibody of the secondary antibody response
IgA: major class of antibody in mucous membranes, in saliva
and tears
IgE: small amount; produces harmful effects such as allergies
IgD: small amount in blood; precise function unknown
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B CELLS AND ANTIBODYMEDIATED IMMUNITY (cont.)
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Antibody molecules produce antibodymediated immunity (humoral immunity)
within plasma
Antibodies resist disease first by
recognizing foreign or abnormal
substances (Figure 21-19)
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Epitopes bind to an antibody molecule’s
antigen-binding sites, which forms an antigenantibody complex that may have several
effects (Figure 21-20)
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B CELLS AND ANTIBODYMEDIATED IMMUNITY (cont.)
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Complement: a component of blood plasma
consisting of several protein compounds
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Complement kills foreign cells by cytolysis or apoptosis
(Figures 21-21 and 21-22)
Complement causes vasodilation, enhances
phagocytosis, and has other functions
Complement activity can also be initiated by innate
immune mechanisms
• Formation by innate immunity is called the alternate
pathway
• Complement protein 3, activated without antigen
stimulation, produces full complement effect by binding to
bacteria or viruses in presence of properdin
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B CELLS AND ANTIBODYMEDIATED IMMUNITY (cont.)
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Primary and secondary responses (Figure
21-23)
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Primary response: initial encounter with a
specific antigen triggers the formation and
release of specific antibodies that reaches its
peak in a few days
Secondary response: a later encounter with
the same antigen triggers a much quicker
response; B memory cells rapidly divide,
producing more plasma cells and thus more
antibodies
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B CELLS AND ANTIBODYMEDIATED IMMUNITY (cont.)
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Clonal selection theory (Figure 21-24)
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The body contains many diverse clones of
cells, each committed by its genes to
synthesize a different antibody
When an antigen enters the body, it selects the
clone whose cells synthesize its antibody and
stimulates them to proliferate and create more
antibody
The clones selected by antigens consist of
lymphocytes and are selected by the shape of
antigen receptors on the lymphocyte’s plasma
membrane
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T CELLS AND CELL-MEDIATED
IMMUNITY
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T cells: lymphocytes that go through the thymus gland
before migrating to the lymph nodes and spleen
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Pre–T cells develop into thymocytes while in the thymus
 Thymocytes stream into the blood and are carried to the T
cell–dependent zones in the spleen and the lymph nodes
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Activation of T cells
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T cells display antigen receptors on their surface membranes
that are similar to antibodies
A T cell is activated when an antigen (presented by an antigenpresenting complex) binds to its receptors (at an
immunological synapse), causing the T cell to divide
repeatedly to form a clone of identical T cells (Figure 21-25)
• Cells of the clone differentiate into effector T cells and
memory T cells
• Effector T cells go to the site where the antigen entered,
bind to antigens, and begin their attack
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T CELLS AND CELL-MEDIATED
IMMUNITY (cont.)
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Functions of T cells
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Cytotoxic T cells: T cells release lymphotoxin to
kill cells (Figure 21-26)
 Helper T cells: regulate the function of B cells,
T cells, phagocytes, and other leukocytes
(Figure 21-27)
 Suppressor T cells: regulatory T cells that
suppress lymphocyte function, thus regulating
immunity and promoting self-tolerance
 T cells function to produce cell-mediated
immunity and help regulate adaptive immunity
in general
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TYPES OF ADAPTIVE IMMUNITY
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Innate immunity (inborn or inherited
immunity): genetic mechanisms put innate
immune mechanisms in place during
development in the womb (Table 21-4)
Adaptive or acquired immunity: resistance
developed after birth; two types:
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Natural immunity results from nondeliberate
exposure to antigens
 Artificial immunity results from deliberate
exposure to antigens, called immunization
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TYPES OF ADAPTIVE IMMUNITY
(cont.)
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Natural and artificial immunity may be
active or passive
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Active immunity: when immune system
responds to a harmful agent regardless of
whether it is natural or artificial; lasts longer
than passive
Passive immunity: immunity developed in
another individual is transferred to an individual
who was not previously immune; temporary but
provides immediate protection
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SUMMARY OF ADAPTIVE
IMMUNITY
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Adaptive immunity is specific immunity targeting specific
antigens
Adaptive immunity involves two classes of lymphocyte: B cells
and T cells (Figure 21-27)
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B cells: antibody-mediated (humoral) immunity
T cells: cell-mediated (cellular) immunity
Adaptive immunity occurs in a series of stages (Figure 21-28)
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Recognition of antigen
Activation of lymphocytes
Effector phase (immune attack)
Decline of antigen causes lymphocyte death (homeostatic
balance)
 Memory cells remain for later response if needed
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B cells and T cells work together in a coordinated system of
adaptive immunity (Figure 21-29)
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THE BIG PICTURE: IMMUNE
SYSTEM AND THE WHOLE BODY
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Immune system regulated to some degree
by nervous and endocrine systems
Agents of the immune system include
blood cells, skin cells, mucosal cells, brain
cells, liver cells, and other types of cells
and their secretions
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