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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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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.
• Macrophages, a specific type of phagocyte
– Can be found migrating through the body
– Can be found in various organs of the
lymphatic system
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• The lymphatic system
– Plays an active role in defending the body from
pathogens
1 Interstitial fluid bathing the
tissues, along with the white
blood cells in it, continually
enters lymphatic capillaries.
Interstitial
fluid
Adenoid
Lymphatic
capillary
2 Fluid inside the
lymphatic capillaries,
called lymph, flows
through lymphatic
vessels throughout
the body.
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
Figure 43.5
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Lymph
node
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.
Antimicrobial Proteins
• Numerous proteins function in innate defense
– By attacking microbes directly of by impeding
their reproduction
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• About 30 proteins make up the complement
system
– Which can cause lysis of invading cells and
help trigger inflammation
• Interferons
– Provide innate defense against viruses and
help activate macrophages
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Inflammatory Response
• In local inflammation, histamine and other
chemicals released from injured cells
– Promote changes in blood vessels that allow
more fluid, more phagocytes, and antimicrobial
proteins to enter the tissues
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• 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.
Figure 43.6
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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.
Natural Killer Cells
• Natural killer (NK) cells
– Patrol the body and attack virus-infected body
cells and cancer cells
– Trigger apoptosis in the cells they attack
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Invertebrate Immune Mechanisms
• Many invertebrates defend themselves from
infection
– By many of the same mechanisms in the
vertebrate innate response
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• 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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• 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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B Cell Receptors for Antigens
• B cell receptors
– Bind to specific, intact antigens
– Are often called membrane antibodies or membrane
immunoglobulins
Antigenbinding
site
Antigenbinding site
Disulfide
bridge
Variable
regions
Light
chain
Constant
regions
C C
Transmembrane
region
Plasma
membrane
Heavy chains
B cell
Figure 43.8a
Cytoplasm of B cell
(a) A B cell receptor consists of two identical heavy
chains and two identical light chains linked by
several disulfide bridges.
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T Cell Receptors for Antigens and the Role of the MHC
• Each T cell receptor
– Consists of two different polypeptide chains
AntigenBinding site
Variable
regions
Constant
regions
V
V
C
C
Transmembrane
region
Plasma
membrane
a chain
b chain
Disulfide bridge
Cytoplasm of T cell
Figure 43.8b
(b) A T cell receptor consists of one
a chain and one b chain linked by
a disulfide bridge.
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T cell
• T cells bind to small fragments of antigens
– That are bound to normal cell-surface proteins
called MHC molecules
• MHC molecules
– Are encoded by a family of genes called the
major histocompatibility complex
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• Infected cells produce MHC molecules
– Which bind to antigen fragments and then are
transported to the cell surface in a process
called antigen presentation
• A nearby T cell
– Can then detect the antigen fragment
displayed on the cell’s surface
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• Depending on their source
– Peptide antigens are handled by different
classes of MHC molecules
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• Class I MHC molecules, found on almost all
nucleated cells of the body
– Display peptide antigens to cytotoxic T cells
Infected cell
Antigen
fragment
1
1 A fragment of
foreign protein
(antigen) inside the
cell associates with
an MHC molecule
and is transported
to the cell surface.
Class I MHC
molecule
2
T cell
receptor
Figure 43.9a
(a) Cytotoxic T cell
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2 The combination of
MHC molecule and
antigen is recognized
by a T cell, alerting it
to the infection.
• Class II MHC molecules, located mainly on
dendritic cells, macrophages, and B cells
– Display antigens to helper T cells
Microbe
1 A fragment of
foreign protein
(antigen) inside the
cell associates with
an MHC molecule
and is transported
to the cell surface.
Antigenpresenting
cell
Antigen
fragment
1
Class II MHC
molecule
2
2 The combination of
MHC molecule and
antigen is recognized
by a T cell, alerting it
to the infection.
Figure 43.9b
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(b)
T cell
receptor
Helper T cell
Lymphocyte Development
• Lymphocytes
– Arise from stem cells in the bone marrow
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• Newly formed lymphocytes are all alike
– But they later develop into B cells or T cells,
depending on where they continue their maturation
Bone marrow
Lymphoid
stem cell
Thymus
T cell
B cell
Blood, lymph, and lymphoid tissues
(lymph nodes, spleen, and others)
Figure 43.10
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Generation of Lymphocyte Diversity by Gene
Rearrangement
• Early in development, random, permanent
gene rearrangement
– Forms functional genes encoding the B or T
cell antigen receptor chains
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• Immunoglobulin gene rearrangement
V4–V39
DNA of
undifferentiated
B cell
V2
V1
V40
V3
J1 J2 J3 J4 J5 Intron
C
1 Deletion of DNA between a V segment
and J segment and joining of the segments
DNA of differentiated
B cell
V1
V3 J5 Intron
V2
C
2 Transcription of resulting permanently rearranged,
functional gene
pre-mRNA
V3 J5
Intron
3
mRNA Cap
V 3 J5
C
RNA processing (removal of intron; addition of cap
and poly (A) tail)
C
Poly (A)
4 Translation
Light-chain polypeptide
Figure 43.11
V
C
Variable Constant
region
region
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B cell receptor
B cell
Testing and Removal of Self-Reactive Lymphocytes
• As B and T cells are maturing in the bone and
thymus
– Their antigen receptors are tested for possible
self-reactivity
• Lymphocytes bearing receptors for antigens
already present in the body
– Are destroyed by apoptosis or rendered
nonfunctional
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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.
• 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
• Concept 43.3: 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
Helper T Cells: A Response to Nearly All Antigens
• Helper T cells produce CD4, a surface protein
– That enhances their binding to class II MHC
molecule–antigen complexes on antigenpresenting cells
• Activation of the helper T cell then occurs
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• Activated helper T cells
– Secrete several different cytokines that
stimulate other lymphocytes
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• The role of helper T cells in acquired immunity
1 After a dendritic cell engulfs and degrades a bacterium, it displays
bacterial antigen fragments (peptides) complexed with a class II
MHC molecule on the cell surface. A specific helper T cell binds
to the displayed complex via its TCR with the aid of CD4. This
interaction promotes secretion of cytokines by the dendritic cell.
Cytotoxic T cell
Dendritic
cell
Bacterium
Peptide antigen
Class II MHC
molecule
Helper T cell
Cell-mediated
immunity
(attack on
infected cells)
TCR
2
3
1 CD4
Dendritic
cell
Cytokines
2 Proliferation of the T cell, stimulated
by cytokines from both the dendritic
cell and the T cell itself, gives rise to
a clone of activated helper T cells
(not shown), all with receptors for the
same MHC–antigen complex.
Figure 43.15
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B cell
3 The cells in this clone
secrete other cytokines
that help activate B cells
and cytotoxic T cells.
Humoral
immunity
(secretion of
antibodies by
plasma cells)
Cytotoxic T Cells: A Response to Infected Cells
and Cancer Cells
• Cytotoxic T cells make CD8
– A surface protein that greatly enhances the
interaction between a target cell and a
cytotoxic T cell
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• Cytotoxic T cells
– Bind to infected cells, cancer cells, and
transplanted tissues
• Binding to a class I MHC complex on an
infected body cell
– Activates a cytotoxic T cell and differentiates it
into an active killer
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• The activated cytotoxic T cell
– Secretes proteins that destroy the infected target
cell
2
1 A specific cytotoxic T cell binds to a
class I MHC–antigen complex on a
target cell via its TCR with the aid of
CD8. This interaction, along with
cytokines from helper T cells, leads to
the activation of the cytotoxic cell.
The activated T cell releases perforin
molecules, which form pores in the
target cell membrane, and proteolytic
enzymes (granzymes), which enter the
target cell by endocytosis.
Cytotoxic T cell
3 The granzymes initiate apoptosis within the
target cells, leading to fragmentation of the
nucleus, release of small apoptotic bodies,
and eventual cell death. The released
cytotoxic T cell can attack other target cells.
Released
cytotoxic
T cell
Perforin
Cancer
cell
Granzymes
1 TCR
Class I MHC
molecule
Target
cell
3
CD8
2
Apoptotic
target cell
Pore
Peptide
antigen
Figure 43.16
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Cytotoxic
T cell
B Cells: A Response to Extracellular Pathogens
• Activation of B cells
– Is aided by cytokines and antigen binding to
helper T cells
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• The clonal selection of B cells
– Generates antibody-secreting plasma cells, the
effector cells of humoral immunity
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1 After a macrophage engulfs and degrades
2
a bacterium, it displays a peptide antigen
complexed with a class II MHC molecule.
A helper T cell that recognizes the displayed
complex is activated with the aid of cytokines
secreted from the macrophage, forming a
clone of activated helper T cells (not shown).
A B cell that has taken up and degraded the
same bacterium displays class II MHC–peptide
antigen complexes. An activated helper T cell
bearing receptors specific for the displayed
antigen binds to the B cell. This interaction,
with the aid of cytokines from the T cell,
activates the B cell.
3 The activated B cell proliferates
and differentiates into memory
B cells and antibody-secreting
plasma cells. The secreted
antibodies are specific for the
same bacterial antigen that
initiated the response.
Bacterium
Macrophage
Peptide
antigen
Class II
MHC
molecule
B cell
2
3
1
TCR
Clone of plasma cells
Endoplasmic
reticulum of
plasma cell
CD4
Cytokines
Helper T cell
Secreted antibody
molecules
Activated
helper T cell
Figure 43.17
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Clone of memory
B cells
Antibody Classes
• The five major classes of antibodies, or
immunoglobulins
– Differ in their distributions and functions within
the body
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• The five classes of immunoglobulins
IgM
(pentamer)
First Ig class produced after initial exposure to
antigen; then its concentration in the blood declines
J chain
IgG
(monomer)
Promotes neutralization and agglutination of
antigens; very effective in complement activation
(see Figure 43.19)
Most abundant Ig class in blood; also present in
tissue fluids
Only Ig class that crosses placenta, thus conferring
passive immunity on fetus
Promotes opsonization, neutralization, and agglutination
of antigens; less effective in complement activation than
IgM (see Figure 43.19)
Present in secretions such as tears, saliva, mucus,
and breast milk
IgA
(dimer)
Secretory
component
J chain
Provides localized defense of mucous membranes by
agglutination and neutralization of antigens (see
Figure 43.19)
Presence in breast milk confers passive immunity on
nursing infant
IgE
(monomer)
IgD
(monomer)
Figure 43.18
Transmembrane
region
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Triggers release from mast cells and basophils of
histamine and other chemicals that cause allergic
reactions (see Figure 43.20)
Present primarily on surface of naive B cells that have
not been exposed to antigens
Acts as antigen receptor in antigen-stimulated
proliferation and differentiation of B cells (clonal
selection)
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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Blood Groups and Transfusions
• Certain antigens on red blood cells
– Determine whether a person has type A, B,
AB, or O blood
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• Antibodies to nonself blood types
– Already exist in the body
• Transfusion with incompatible blood
– Leads to destruction of the transfused cells
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• Recipient-donor combinations
– Can be fatal or safe
Table 43.1
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• Another red blood cell antigen, the Rh factor
– Creates difficulties when an Rh-negative
mother carries successive Rh-positive fetuses
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Tissue and Organ Transplants
• MHC molecules
– Are responsible for stimulating the rejection of
tissue grafts and organ transplants
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• The chances of successful transplantation are
increased
– If the donor and recipient MHC tissue types
are well matched
– If the recipient is given immunosuppressive
drugs
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• Lymphocytes in bone marrow transplants
– May cause a graft versus host reaction in
recipients
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• Concept 43.5: Exaggerated, self-directed, or
diminished immune responses can cause
disease
• If the delicate balance of the immune system is
disrupted
– The effects on the individual can range from
minor to often fatal consequences
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Allergies
• Allergies are exaggerated (hypersensitive)
responses
– To certain antigens called allergens
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• In localized allergies such as hay fever
– IgE antibodies produced after first exposure to
an allergen attach to receptors on mast cells
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• The next time the allergen enters the body
– It binds to mast cell–associated IgE molecules
• The mast cells then release histamine and
other mediators
– That cause vascular changes and typical
symptoms
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• 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.
Figure 43.20
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• An acute allergic response sometimes leads to
anaphylactic shock
– A whole-body, life-threatening reaction that
can occur within seconds of exposure to an
allergen
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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
include
– Systemic lupus erythematosus
– Multiple sclerosis
– Insulin-dependent diabetes
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Immunodeficiency Diseases
• An inborn or primary immunodeficiency
– Results from hereditary or congenital defects
that prevent proper functioning of innate,
humoral, and/or cell-mediated defenses
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• An acquired or secondary immunodeficiency
– Results from exposure to various chemical and
biological agents
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Inborn (Primary) Immunodeficiencies
• In severe combined immunodeficiency (SCID)
– Both the humoral and cell-mediated branches
of acquired immunity fail to function
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Acquired (Secondary) Immunodeficiencies
• Acquired immunodeficiencies
– Range from temporary states to chronic
diseases
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Stress and the Immune System
• Growing evidence shows
– That physical and emotional stress can harm
immunity
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• Acquired Immunodeficiency Syndrome (AIDS)
• People with AIDS
– Are highly susceptible to opportunistic
infections and cancers that take advantage of
an immune system in collapse
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• Because AIDS arises from the loss of helper T
cells
– Both humoral and cell-mediated immune
responses are impaired
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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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