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Chapter 43
The Immune System
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Reconnaissance, Recognition, and
Response
• An animal must defend itself
– From the many dangerous pathogens it may
encounter in the environment
• Two major kinds of defense have evolved that
counter these threats
– Innate immunity and acquired immunity
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• Innate immunity
– Is present before any exposure to pathogens
and is effective from the time of birth
– Involves nonspecific responses to pathogens
Figure 43.1
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3m
• Acquired immunity, also called adaptive
immunity
– Develops only after exposure to inducing
agents such as microbes, toxins, or other
foreign substances
– Involves a very specific response to pathogens
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• A summary of innate and acquired immunity
INNATE IMMUNITY
Rapid responses to a
broad range of microbes
External defenses
Invading
microbes
(pathogens)
Internal defenses
Skin
Phagocytic cells
Mucous membranes
Antimicrobial proteins
Secretions
Inflammatory response
Figure 43.2
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ACQUIRED IMMUNITY
Slower responses to
specific microbes
Natural killer cells
Humoral response
(antibodies)
Cell-mediated response
(cytotoxic
lymphocytes)
• Concept 43.1: Innate immunity provides broad
defenses against infection
• A pathogen that successfully breaks through
an animal’s external defenses
– Soon encounters several innate cellular and
chemical mechanisms that impede its attack
on the body
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External Defenses
• Intact skin and mucous membranes
– Form physical barriers that bar the entry of
microorganisms and viruses
• Certain cells of the mucous membranes
produce mucus
– A viscous fluid that traps microbes and other
particles
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• In the trachea, ciliated epithelial cells
– Sweep mucus and any entrapped microbes
upward, preventing the microbes from entering
the lungs
10m
Figure 43.3
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• Secretions of the skin and mucous membranes
– Provide an environment that is often hostile to
microbes
• Secretions from the skin
– Give the skin a pH between 3 and 5, which is
acidic enough to prevent colonization of many
microbes
– Also include proteins such as lysozyme, an
enzyme that digests the cell walls of many
bacteria
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Internal Cellular and Chemical Defenses
• Internal cellular defenses
– Depend mainly on phagocytosis
• Phagocytes, types of white blood cells
– Ingest invading microorganisms
– Initiate the inflammatory response
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Figure 43.6 Major events in the local inflammatory response
Blood clot
Pin
Pathogen
Macrophage
Chemical signals
Phagocytic cells
Capillary
Blood
clotting
elements
Phagocytosis
Red blood cell
1 Chemical signals released
by activated macrophages
and mast cells at the injury
site cause nearby capillaries
to widen and become more
permeable.
2 Fluid, antimicrobial proteins,
and clotting elements move
from the blood to the site.
Clotting begins.
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3 Chemokines released by various
kinds of cells attract more
phagocytic cells from the blood
to the injury site.
4 Neutrophils and macrophages
phagocytose pathogens and
cell debris at the site, and the
tissue heals.
Phagocytic Cells
• Phagocytes attach to their prey via surface receptors
– And engulf them, forming a vacuole that fuses with a
lysosome
1 Pseudopodia
surround
microbes.
Microbes
2 Microbes
are engulfed
into cell.
MACROPHAGE
3 Vacuole
containing
microbes
forms.
Vacuole
Lysosome
containing
enzymes
4 Vacuole
and lysosome
fuse.
5 Toxic
compounds
and lysosomal
enzymes
destroy microbes.
Figure 43.4
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6 Microbial
debris is
released by
exocytosis.
• Concept 43.2: In acquired immunity,
lymphocytes provide specific defenses against
infection
• Acquired immunity
– Is the body’s second major kind of defense
– Involves the activity of lymphocytes
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Figure 43.5 The human lymphatic system
1 Interstitial fluid bathing the
tissues, along with the white
blood cells in it, continually
enters lymphatic capillaries.
Interstitial
fluid
Lymphatic
capillary
2 Fluid inside the
lymphatic capillaries,
called lymph, flows
through lymphatic
vessels throughout
the body.
Adenoid
Tonsil
4 Lymphatic vessels
return lymph to the
blood via two large
ducts that drain into
veins near the
shoulders.
Lymph
nodes
Blood
capillary
Spleen
Peyer’s patches
(small intestine)
Tissue
cells
Lymphatic
vessel
Appendix
Lymphatic
vessels
Lymph
node
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Masses of
lymphocytes and
macrophages
3 Within lymph nodes,
microbes and foreign
particles present in
the circulating lymph
encounter macrophages, dendritic cells,
and lymphocytes,
which carry out
various defensive
actions.
• An antigen is any foreign molecule
– That is specifically recognized by lymphocytes
and elicits a response from them
• A lymphocyte actually recognizes and binds
– To just a small, accessible portion of the antigen
called an epitope
Antigenbinding
sites
Antibody A
Antigen
Antibody B
Antibody C
Figure 43.7
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Epitopes
(antigenic
determinants)
Antigen Recognition by Lymphocytes
• The vertebrate body is populated by two main
types of lymphocytes
– B lymphocytes (B cells) and T lymphocytes
(T cells)
– Which circulate through the blood
• The plasma membranes of both B cells
and T cells
– Have about 100,000 antigen receptor that all
recognize the same epitope
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Clonal Selection of Lymphocytes
• In a primary immune response
– Binding of antigen to a mature lymphocyte
induces the lymphocyte’s proliferation and
differentiation, a process called clonal
selection
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• Clonal selection of B cells
– Generates a clone of short-lived activated effector
cells and a clone of long-lived memory cells
Antigen molecules
B cells that
differ in
antigen
specificity
Antigen
receptor
Antigen molecules
bind to the antigen
receptors of only one
of the three B cells
shown.
The selected B cell
proliferates, forming
a clone of identical
cells bearing
receptors for the
selecting antigen.
Some proliferating cells
develop into long-lived
memory cells that can
respond rapidly upon
subsequent exposure
to the same antigen.
Antibody
molecules
Clone of memory cells
Figure 43.12
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Clone of plasma cells
Some proliferating
cells develop into
short-lived plasma
cells that secrete
antibodies specific
for the antigen.
Primary and Secondary Immune Responses
• In the secondary immune response, memory
cells facilitate a faster, more efficient response
1 Day 1: First
2 Primary
exposure to
response to
antigen A
antigen A
produces antibodies to A
3 Day 28:
Second exposure
to antigen A; first
exposure to
antigen B
4 Secondary response to antigen A produces antibodies
to A; primary response to antigen B produces antibodies to B
Antibody concentration
(arbitrary units)
104
103
102
Antibodies
to A
Antibodies
to B
101
100
Figure 43.13
0
7
14
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21
28
35
Time (days)
42
49
56
Humoral and cell-mediated immunity defend
against different types of threats
• Acquired immunity includes two branches
– The humoral immune response involves the
activation and clonal selection of B cells,
resulting in the production of secreted
antibodies
– The cell-mediated immune response involves
the activation and clonal selection of cytotoxic
T cells
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• The roles of the major participants in the
acquired immune response
Cell-mediated immune response
Humoral immune response
First exposure to antigen
Intact antigens
Antigens engulfed and
displayed by dendritic cells
Antigens displayed
by infected cells
Activate
Activate
Activate
B cell
Gives rise to
Plasma
cells
Figure 43.14
Memory
B cells
Helper
T cell
Gives rise to
Active and
memory
helper
T cells
Secrete antibodies that defend against
pathogens and toxins in extracellular fluid
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Secreted
cytokines
activate
Cytotoxic
T cell
Gives rise to
Memory
cytotoxic
T cells
Active
cytotoxic
T cells
Defend against infected cells, cancer
cells, and transplanted tissues
Figure 43.10 Overview of lymphocyte development
Bone marrow
Thymus
Lymphoid
stem cell
T cell
B cell
Blood, lymph, and lymphoid tissues
(lymph nodes, spleen, and others)
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Antibody-Mediated Disposal of Antigens
• The binding of antibodies to antigens
– Is also the basis of several antigen disposal
mechanisms
– Leads to elimination of microbes by
phagocytosis and complement-mediated lysis
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Antibody-mediated mechanisms of antigen
disposal
Binding of antibodies to antigens
inactivates antigens by
Viral neutralization
(blocks binding to host)
and opsonization (increases
phagocytosis)
Agglutination of
antigen-bearing particles,
such as microbes
Precipitation of
soluble antigens
Complement
proteins
Bacteria
Virus
Activation of complement system
and pore formation
MAC
Pore
Soluble
antigens
Bacterium
Figure 43.19
Foreign cell
Enhances
Leads to
Phagocytosis
Cell lysis
Macrophage
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Active and Passive Immunization
• Active immunity
– Develops naturally in response to an infection
– Can also develop following immunization, also
called vaccination
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• In immunization
– A nonpathogenic form of a microbe or part of a
microbe elicits an immune response to an
immunological memory for that microbe
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• Passive immunity, which provides immediate,
short-term protection
– Is conferred naturally when IgG crosses the
placenta from mother to fetus or when IgA
passes from mother to infant in breast milk
– Can be conferred artificially by injecting
antibodies into a nonimmune person
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• Concept 43.4: The immune system’s ability to
distinguish self from nonself limits tissue
transplantation
• The immune system
– Can wage war against cells from other
individuals
• Transplanted tissues
– Are usually destroyed by the recipient’s
immune system
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• Recipient-donor combinations
– Can be fatal or safe
Table 43.1
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Figure 43.20 Mast cells, IgE, and the allergic response
IgE
Allergen
Histamine
1
3
2
Granule
Mast cell
1 IgE antibodies produced in 2 On subsequent exposure to the 3 Degranulation of the cell,
triggered by cross-linking of
response to initial exposure
same allergen, IgE molecules
adjacent IgE molecules,
to an allergen bind to
attached to a mast cell recogreleases histamine and other
receptors or mast cells.
nize and bind the allergen.
chemicals, leading to allergy
symptoms.
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Autoimmune Diseases
• In individuals with autoimmune diseases
– The immune system loses tolerance for self
and turns against certain molecules of the
body
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Rheumatoid arthritis
• Is an autoimmune disease that leads to
damage and painful inflammation of the
cartilage and bone of joints
Figure 43.21
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Other examples of autoimmune diseases:
• Systemic lupus erythematosus (lupus)
– Antibodies against a wide range of self
molecules
• Multiple sclerosis
– Destruction of myelin sheath surrounding
nerves
• Insulin-dependent diabetes
– Insulin- producing pancreas cells are targeted
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Acquired Immunodeficiency Syndrome (AIDS)
• Because AIDS arises from the loss of helper T
cells
– Both humoral and cell-mediated immune
responses are impaired
• People with AIDS
– Are highly susceptible to opportunistic
infections and cancers that take advantage of
an immune system in collapse
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• The loss of helper T cells
– Results from infection by the human
immunodeficiency virus (HIV)
Figure 43.22
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1µm
• The spread of HIV
– Has become a worldwide problem
• The best approach for slowing the spread of
HIV
– Is educating people about the practices that
transmit the virus
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