the immune system … · • the immune system recognizes bacteria and fungi by structures on their...
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
Biology Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 43 Chapter 43
The Immune System
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Overview: Reconnaissance, Recognition, and Response
• Barriers help an animal to defend itself from the many dangerous pathogens it may encounter
• The immune system recognizes foreign bodies and responds with the production of immune cells and proteins
• Two major kinds of defense have evolved: innate immunity and acquired immunity
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Fig. 43-1
1.5 µm
https://www.youtube.com/watch?v=HiJ_tcQJUwM
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• Innate immunity is present before any exposure to pathogens and is effective from the time of birth
• It involves nonspecific responses to pathogens
• Innate immunity consists of external barriers plus internal cellular and chemical defenses
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• Acquired immunity, or adaptive immunity, develops after exposure to agents such as microbes, toxins, or other foreign substances
• It involves a very specific response to pathogens
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Fig. 43-2
INNATE IMMUNITY
Recognition of traits shared by broad ranges of pathogens, using a small set of receptors
•
• Rapid response
• Recognition of traits specific to particular pathogens, using a vast array of receptors
• Slower response
ACQUIRED IMMUNITY
Pathogens (microorganisms
and viruses)
Barrier defenses: Skin Mucous membranes Secretions
Internal defenses: Phagocytic cells Antimicrobial proteins Inflammatory response Natural killer cells
Humoral response: Antibodies defend against infection in body fluids.
Cell-mediated response: Cytotoxic lymphocytes defend against infection in body cells.
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Concept 43.1: In innate immunity, recognition and response rely on shared traits of pathogens
• Both invertebrates and vertebrates depend on innate immunity to fight infection
• Vertebrates also develop acquired immune defenses
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Innate Immunity of Invertebrates
• In insects, an exoskeleton made of chitin forms the first barrier to pathogens
• The digestive system is protected by low pH and lysozyme, an enzyme that digests microbial cell walls
• Hemocytes circulate within hemolymph and carry out phagocytosis, the ingestion and digestion of foreign substances including bacteria
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Fig. 43-3
Microbes
PHAGOCYTIC CELL
Vacuole Lysosome containing enzymes
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• Hemocytes also secrete antimicrobial peptides that disrupt the plasma membranes of bacteria
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Fig. 43-4
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• The immune system recognizes bacteria and fungi by structures on their cell walls
• An immune response varies with the class of pathogen encountered
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Fig. 43-5 RESULTS
% s
urvi
val
Wild type
Fruit fly survival after infection by N. crassa fungi
100
75
50
25
0 0 24 72 48 96 120
100
75
50
25
0 0 24 72 48 96 120
Hours post-infection
Fruit fly survival after infection by M. luteus bacteria
Hours post-infection
% s
urvi
val
Mutant + drosomycin
Mutant + defensin Mutant
Wild type
Mutant + drosomycin
Mutant + defensin
Mutant
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Fig. 43-5a
RESULTS
% s
urvi
val
Wild type
Fruit fly survival after infection by N. crassa fungi
100
75
50
25
0 0 24 72 48 96 120
Hours post-infection
Mutant + drosomycin
Mutant + defensin Mutant
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Fig. 43-5b
100
75
50
25
0 0 24 72 48 96 120
Fruit fly survival after infection by M. luteus bacteria Hours post-infection
% s
urvi
val
Wild type
Mutant + drosomycin
Mutant + defensin
Mutant
RESULTS
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Innate Immunity of Vertebrates
• The immune system of mammals is the best understood of the vertebrates
• Innate defenses include barrier defenses, phagocytosis, antimicrobial peptides
• Additional defenses are unique to vertebrates: the inflammatory response and natural killer cells
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Barrier Defenses
• Barrier defenses include the skin and mucous membranes of the respiratory, urinary, and reproductive tracts
• Mucus traps and allows for the removal of microbes
• Many body fluids including saliva, mucus, and tears are hostile to microbes
• The low pH of skin and the digestive system prevents growth of microbes
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Cellular Innate Defenses
• White blood cells (leukocytes) engulf pathogens in the body
• Groups of pathogens are recognized by TLR, Toll-like receptors
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Fig. 43-6
EXTRACELLULAR FLUID Lipopolysaccharide
Flagellin TLR4
TLR5
Helper protein
TLR9
TLR3
WHITE BLOOD CELL
VESICLE
CpG DNA
ds RNA
Inflammatory responses
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• A white blood cell engulfs a microbe, then fuses with a lysosome to destroy the microbe
• There are different types of phagocytic cells:
– Neutrophils engulf and destroy microbes
– Macrophages are part of the lymphatic system and are found throughout the body
– Eosinophils discharge destructive enzymes
– Dendritic cells stimulate development of acquired immunity
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Fig. 43-7
Adenoid Tonsil
Lymph nodes Spleen
Peyer’s patches (small intestine)
Appendix
Lymphatic vessels Lymph
node Masses of defensive cells
Blood capillary
Lymphatic vessel
Tissue cells
Interstitial fluid
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Antimicrobial Peptides and Proteins
• Peptides and proteins function in innate defense by attacking microbes directly or impeding their reproduction
• Interferon proteins provide innate defense against viruses and help activate macrophages
• About 30 proteins make up the complement system, which causes lysis of invading cells and helps trigger inflammation
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Inflammatory Responses
• Following an injury, mast cells release histamine, which promotes changes in blood vessels; this is part of the inflammatory response
• These changes increase local blood supply and allow more phagocytes and antimicrobial proteins to enter tissues
• Pus, a fluid rich in white blood cells, dead microbes, and cell debris, accumulates at the site of inflammation
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Fig. 43-8-1
Pathogen Splinter
Macrophage
Mast cell
Chemical signals
Capillary
Phagocytic cell Red blood cells
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Fig. 43-8-2
Pathogen Splinter
Macrophage
Mast cell
Chemical signals
Capillary
Phagocytic cell Red blood cells
Fluid
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Fig. 43-8-3
Pathogen Splinter
Macrophage
Mast cell
Chemical signals
Capillary
Phagocytic cell Red blood cells
Fluid
Phagocytosis
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• Inflammation can be either local or systemic (throughout the body)
• Fever is a systemic inflammatory response triggered by pyrogens released by macrophages, and toxins from pathogens
• Septic shock is a life-threatening condition caused by an overwhelming inflammatory response
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Natural Killer Cells
• All cells in the body (except red blood cells) have a class 1 MHC protein on their surface
• Cancerous or infected cells no longer express this protein; natural killer (NK) cells attack these damaged cells
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Innate Immune System Evasion by Pathogens
• Some pathogens avoid destruction by modifying their surface to prevent recognition or by resisting breakdown following phagocytosis
• Tuberculosis (TB) is one such disease and kills more than a million people a year
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Concept 43.2: In acquired immunity, lymphocyte receptors provide pathogen-specific recognition
• White blood cells called lymphocytes recognize and respond to antigens, foreign molecules
• Lymphocytes that mature in the thymus above the heart are called T cells, and those that mature in bone marrow are called B cells
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• Lymphocytes contribute to immunological memory, an enhanced response to a foreign molecule encountered previously
• Cytokines are secreted by macrophages and dendritic cells to recruit and activate lymphocytes
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Acquired Immunity: An Overview
• B cells and T cells have receptor proteins that can bind to foreign molecules
• Each individual lymphocyte is specialized to recognize a specific type of molecule
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Antigen Recognition by Lymphocytes
• An antigen is any foreign molecule to which a lymphocyte responds
• A single B cell or T cell has about 100,000 identical antigen receptors
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Fig. 43-9
Antigen- binding site
Antigen- binding site
Antigen- binding site
Disulfide bridge
Variable regions Constant regions
Transmembrane region
Plasma membrane
Light chain
Heavy chains
T cell
chain chain Disulfide bridge
Cytoplasm of T cell
(b) T cell receptor
Cytoplasm of B cell
(a) B cell receptor
B cell
C C C C
V V
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Fig. 43-9a
Antigen- binding site
Antigen- binding site
Disulfide bridge
Variable regions
Constant regions
Transmembrane region
Plasma membrane
Light chain
Heavy chains
Cytoplasm of B cell
(a) B cell receptor
B cell
C C
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Fig. 43-9b
Antigen- binding site
Variable regions
Constant regions
Transmembrane region
Plasma membrane
T cell
chain chain Disulfide bridge
Cytoplasm of T cell
(b) T cell receptor
C C
V V
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• All antigen receptors on a single lymphocyte recognize the same epitope, or antigenic determinant, on an antigen
• B cells give rise to plasma cells, which secrete proteins called antibodies or immunoglobulins
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Fig. 43-10
Antigen-binding sites
Antigen- binding sites
Epitopes (antigenic determinants)
Antigen
Antibody B
Antibody C Antibody A
C C
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The Antigen Receptors of B Cells and T Cells
• B cell receptors bind to specific, intact antigens
• The B cell receptor consists of two identical heavy chains and two identical light chains
• The tips of the chains form a constant (C) region, and each chain contains a variable (V) region, so named because its amino acid sequence varies extensively from one B cell to another
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• Secreted antibodies, or immunoglobulins, are structurally similar to B cell receptors but lack transmembrane regions that anchor receptors in the plasma membrane
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• Each T cell receptor consists of two different polypeptide chains
• The tips of the chain form a variable (V) region; the rest is a constant (C) region
• T cells can bind to an antigen that is free or on the surface of a pathogen
Video: T Cell Receptors
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• T cells bind to antigen fragments presented on a host cell
• These antigen fragments are bound to cell-surface proteins called MHC molecules
• MHC molecules are so named because they are encoded by a family of genes called the major histocompatibility complex
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The Role of the MHC
• In infected cells, MHC molecules bind and transport antigen fragments to the cell surface, a process called antigen presentation
• A nearby T cell can then detect the antigen fragment displayed on the cell’s surface
• Depending on their source, peptide antigens are handled by different classes of MHC molecules
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Fig. 43-11
Antigen
Top view: binding surface exposed to antigen receptors
Plasma membrane of infected cell
Antigen
Class I MHC molecule
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• Class I MHC molecules are found on almost all nucleated cells of the body
• They display peptide antigens to cytotoxic T cells
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Fig. 43-12
Infected cell
Antigen fragment
Class I MHC molecule
T cell receptor
(a)
Antigen associates with MHC molecule
T cell recognizes combination
Cytotoxic T cell (b) Helper T cell
T cell receptor
Class II MHC molecule
Antigen fragment
Antigen- presenting cell
Microbe 1
1 1
2 2 2
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• Class II MHC molecules are located mainly on dendritic cells, macrophages, and B cells
• Dendritic cells, macrophages, and B cells are antigen-presenting cells that display antigens to cytotoxic T cells and helper T cells
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Lymphocyte Development
• The acquired immune system has three important properties:
– Receptor diversity
– A lack of reactivity against host cells
– Immunological memory
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Generation of Lymphocyte Diversity by Gene Rearrangement
• Differences in the variable region account for specificity of antigen receptors
• The immunoglobulin (Ig) gene encodes one chain of the B cell receptor
• Many different chains can be produced from the same Ig chain gene by rearrangement of the DNA
• Rearranged DNA is transcribed and translated and the antigen receptor formed
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Fig. 43-13
DNA of undifferentiated B cell
1
DNA of differentiated B cell
pre-mRNA
mRNA
Light-chain polypeptide
Variable region
Constant region
Translation
B cell
B cell receptor
RNA processing
Transcription
DNA deleted between randomly selected V and J segments
Functional gene
V37 V38
V39 V40
J1 J2
J3 J4
C J5 Intron
V37 V38
V39 C J5
Intron
V39 C J5
Intron
V39 C J5
Poly-A tail Cap
C V
V V
V V
C C
C C
2
3
4
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Origin of Self-Tolerance
• Antigen receptors are generated by random rearrangement of DNA
• As lymphocytes mature in bone marrow or the thymus, they are tested for self-reactivity
• Lymphocytes with receptors specific for the body’s own molecules are destroyed by apoptosis, or rendered nonfunctional
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Amplifying Lymphocytes by Clonal Selection
• In the body there are few lymphocytes with antigen receptors for any particular epitope
• The binding of a mature lymphocyte to an antigen induces the lymphocyte to divide rapidly
• This proliferation of lymphocytes is called clonal selection
• Two types of clones are produced: short-lived activated effector cells and long-lived memory cells
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Fig. 43-14
B cells that differ in antigen specificity
Antibody molecules
Antigen receptor
Antigen molecules
Clone of memory cells Clone of plasma cells
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• The first exposure to a specific antigen represents the primary immune response
• During this time, effector B cells called plasma cells are generated, and T cells are activated to their effector forms
• In the secondary immune response, memory cells facilitate a faster, more efficient response
Animation: Role of B Cells
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Fig. 43-15
Antibodies to A Antibodies
to B
Secondary immune response to antigen A produces antibodies to A; primary immune response to antigen B produces antibodies to B.
Primary immune response to antigen A produces antibodies to A.
Ant
ibod
y co
ncen
trat
ion
(arb
itrar
y un
its)
Exposure to antigen A
Exposure to antigens A and B Time (days)
104
103
102
101
100 0 7 14 21 28 35 42 49 56
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Concept 43.3: Acquired immunity defends against infection of body cells and fluids
• Acquired immunity has two branches: the humoral immune response and the cell-mediated immune response
• Humoral immune response involves activation and clonal selection of B cells, resulting in production of secreted antibodies
• Cell-mediated immune response involves activation and clonal selection of cytotoxic T cells
• Helper T cells aid both responses
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Fig. 43-16 Humoral (antibody-mediated) immune response
B cell
Plasma cells
Cell-mediated immune response
Key
Stimulates Gives rise to
+
+
+ +
+
+
+ Memory B cells
Antigen (1st exposure)
Engulfed by
Antigen- presenting cell
Memory Helper T cells
Helper T cell Cytotoxic T cell
Memory Cytotoxic T cells
Active Cytotoxic T cells
Antigen (2nd exposure)
Secreted antibodies
Defend against extracellular pathogens by binding to antigens, thereby neutralizing pathogens or making them better targets for phagocytes and complement proteins.
Defend against intracellular pathogens and cancer by binding to and lysing the infected cells or cancer cells.
+
+ +
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Fig. 43-16a
Key Stimulates Gives rise to
+
Memory Helper T cells
Antigen- presenting cell
Helper T cell
Engulfed by
Antigen (1st exposure)
+
+
+
+ +
+
Defend against extracellular pathogens
Memory B cells
Antigen (2nd exposure) Plasma cells
B cell
Secreted antibodies
Humoral (antibody-mediated) immune response
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Fig. 43-16b Cell-mediated immune response
Defend against intracellular pathogens
Active Cytotoxic T cells
Memory Cytotoxic T cells
Memory Helper T cells
Antigen- presenting cell
Antigen (2nd exposure)
Helper T cell
Engulfed by
Antigen (1st exposure)
Cytotoxic T cell
Key Stimulates Gives rise to
+
+
+
+
+ +
+
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Helper T Cells: A Response to Nearly All Antigens
• A surface protein called CD4 binds the class II MHC molecule
• This binding keeps the helper T cell joined to the antigen-presenting cell while activation occurs
• Activated helper T cells secrete cytokines that stimulate other lymphocytes
Animation: Helper T Cells
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Fig. 43-17
Antigen- presenting cell
Peptide antigen
Cell-mediated immunity (attack on
infected cells)
Class II MHC molecule CD4 TCR (T cell receptor)
Helper T cell
Humoral immunity
(secretion of antibodies by plasma cells) Cytotoxic T cell
Cytokines
B cell
Bacterium
+
+ +
+
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Cytotoxic T Cells: A Response to Infected Cells
• Cytotoxic T cells are the effector cells in cell-mediated immune response
• Cytotoxic T cells make CD8, a surface protein that greatly enhances interaction between a target cell and a cytotoxic T cell
• Binding to a class I MHC complex on an infected cell activates a cytotoxic T cell and makes it an active killer
• The activated cytotoxic T cell secretes proteins that destroy the infected target cell
Animation: Cytotoxic T Cells
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Fig. 43-18-1
Cytotoxic T cell
Perforin Granzymes
TCR
CD8
Class I MHC molecule
Target cell
Peptide antigen
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Fig. 43-18-2
Cytotoxic T cell
Perforin Granzymes
TCR
CD8
Class I MHC molecule
Target cell
Peptide antigen
Pore
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Fig. 43-18-3
Cytotoxic T cell
Perforin Granzymes
TCR
CD8
Class I MHC molecule
Target cell
Peptide antigen
Pore
Released cytotoxic T cell
Dying target cell
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B Cells: A Response to Extracellular Pathogens
• The humoral response is characterized by secretion of antibodies by B cells
• Activation of B cells is aided by cytokines and antigen binding to helper T cells
• Clonal selection of B cells generates antibody-secreting plasma cells, the effector cells of humoral immunity
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Fig. 43-19
Antigen-presenting cell
Endoplasmic reticulum of plasma cell
Secreted antibody molecules
Bacterium
B cell
Peptide antigen
Class II MHC molecule TCR CD4
Helper T cell Activated helper T cell
Cytokines
Clone of memory B cells
Clone of plasma cells
2 µm
+
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Fig. 43-19-1
Antigen-presenting cell
Bacterium
Peptide antigen
Class II MHC molecule TCR CD4
Helper T cell
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Fig. 43-19-2
Antigen-presenting cell
Bacterium
Peptide antigen
Class II MHC molecule TCR CD4
Helper T cell
B cell
Activated helper T cell
Cytokines
+
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Fig. 43-19-3
Antigen-presenting cell
Bacterium
Peptide antigen
Class II MHC molecule TCR CD4
Helper T cell
B cell
Activated helper T cell
Cytokines
+ Secreted antibody molecules
Clone of memory B cells
Clone of plasma cells
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Fig. 43-19a
Endoplasmic reticulum of plasma cell
2 µm
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Antibody Classes
• The five major classes of antibodies, or immunoglobulins, differ in distribution and function
• Polyclonal antibodies are the products of many different clones of B cells following exposure to a microbial antigen
• Monoclonal antibodies are prepared from a single clone of B cells grown in culture
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Fig. 43-20 Class of Immuno-
globulin (Antibody)
IgG (monomer)
IgM (pentamer)
J chain
IgA (dimer)
IgE (monomer)
IgD (monomer)
Trans- membrane region
J chain
Secretory component
Distribution Function
First Ig class produced after initial exposure to antigen; then its concentration in the blood declines
Promotes neutraliza- tion and cross- linking of antigens; very effective in complement system activation
Present in secretions such as tears, saliva, mucus, and breast milk
Only Ig class that crosses placenta, thus conferring passive immunity on fetus
Triggers release from mast cells and basophils of hista- mine and other chemicals that cause allergic reactions
Present primarily on surface of B cells that have not been exposed to antigens
Acts as antigen receptor in the antigen-stimulated proliferation and differentiation of B cells (clonal selection)
Most abundant Ig class in blood; also present in tissue fluids
Promotes opsoniza- tion, neutralization, and cross-linking of antigens; less effec- tive in activation of complement system than IgM
Provides localized defense of mucous membranes by cross-linking and neutralization of antigens
Presence in breast milk confers passive immunity on nursing infant
Present in blood at low concen- trations
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Fig. 43-20a
Distribution Class of Immuno- globulin (Antibody)
IgM (pentamer)
J chain
First Ig class produced after initial exposure to antigen; then its concentration in the blood declines
Promotes neutraliza- tion and cross- linking of antigens; very effective in complement system activation
Function
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Fig. 43-20b
Distribution Function Class of Immuno- globulin (Antibody)
IgG (monomer)
Most abundant Ig class in blood; also present in tissue fluids
Promotes opsoniza- tion, neutralization, and cross-linking of antigens; less effec- tive in activation of complement system than IgM
Only Ig class that crosses placenta, thus conferring passive immunity on fetus
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Fig. 43-20c
Distribution Function Class of Immuno- globulin (Antibody)
IgA (dimer)
J chain
Secretory component
Present in secretions such as tears, saliva, mucus, and breast milk
Provides localized defense of mucous membranes by cross-linking and neutralization of antigens
Presence in breast milk confers passive immunity on nursing infant
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Fig. 43-20d
Distribution Function Class of Immuno- globulin (Antibody)
IgE (monomer)
Present in blood at low concen- trations
Triggers release from mast cells and basophils of hista- mine and other chemicals that cause allergic reactions
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Fig. 43-20e
Distribution Function Class of Immuno- globulin (Antibody)
IgD (monomer)
Trans- membrane region
Present primarily on surface of B cells that have not been exposed to antigens
Acts as antigen receptor in the antigen-stimulated proliferation and differentiation of B cells (clonal selection)
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The Role of Antibodies in Immunity
• Neutralization occurs when a pathogen can no longer infect a host because it is bound to an antibody
• Opsonization occurs when antibodies bound to antigens increase phagocytosis
• Antibodies together with proteins of the complement system generate a membrane attack complex and cell lysis
Animation: Antibodies
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Fig. 43-21
Viral neutralization
Virus
Opsonization
Bacterium
Macrophage
Activation of complement system and pore formation
Complement proteins
Formation of membrane attack complex
Flow of water and ions
Pore
Foreign cell
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Fig. 43-21a
Viral neutralization
Virus
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Fig. 43-21b
Opsonization
Bacterium
Macrophage
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Fig. 43-21c
Activation of complement system and pore formation
Complement proteins
Formation of membrane attack complex
Flow of water and ions
Pore
Foreign cell
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Active and Passive Immunization
• Active immunity develops naturally in response to an infection
• It can also develop following immunization, also called vaccination
• In immunization, a nonpathogenic form of a microbe or part of a microbe elicits an immune response to an immunological memory
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• Passive immunity provides immediate, short-term protection
• It is conferred naturally when IgG crosses the placenta from mother to fetus or when IgA passes from mother to infant in breast milk
• It can be conferred artificially by injecting antibodies into a nonimmune person
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Fig. 43-22
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Immune Rejection
• Cells transferred from one person to another can be attacked by immune defenses
• This complicates blood transfusions or the transplant of tissues or organs
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Blood Groups
• Antigens on red blood cells determine whether a person has blood type A (A antigen), B (B antigen), AB (both A and B antigens), or O (neither antigen)
• Antibodies to nonself blood types exist in the body
• Transfusion with incompatible blood leads to destruction of the transfused cells
• Recipient-donor combinations can be fatal or safe
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Tissue and Organ Transplants
• MHC molecules are different among genetically nonidentical individuals
• Differences in MHC molecules stimulate rejection of tissue grafts and organ transplants
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• Chances of successful transplantation increase if donor and recipient MHC tissue types are well matched
• Immunosuppressive drugs facilitate transplantation
• Lymphocytes in bone marrow transplants may cause the donor tissue to reject the recipient
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Concept 43.4: Disruption in immune system function can elicit or exacerbate disease
• Some pathogens have evolved to diminish the effectiveness of host immune responses
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Exaggerated, Self-Directed, and Diminished Immune Responses
• If the delicate balance of the immune system is disrupted, effects range from minor to often fatal
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Allergies
• Allergies are exaggerated (hypersensitive) responses to antigens called allergens
• 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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Fig. 43-23
Allergen
IgE
Granule Mast cell
Histamine
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• The next time the allergen enters the body, it binds to mast cell–associated IgE molecules
• Mast cells release histamine and other mediators that cause vascular changes leading to typical allergy symptoms
• An acute allergic response can lead to anaphylactic shock, a life-threatening reaction that can occur within seconds of allergen exposure
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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
• Autoimmune diseases include systemic lupus erythematosus, rheumatoid arthritis, insulin-dependent diabetes mellitus, and multiple sclerosis
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Fig. 43-24
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Exertion, Stress, and the Immune System
• Moderate exercise improves immune system function
• Psychological stress has been shown to disrupt hormonal, nervous, and immune systems
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Immunodeficiency Diseases
• Inborn immunodeficiency results from hereditary or developmental defects that prevent proper functioning of innate, humoral, and/or cell-mediated defenses
• Acquired immunodeficiency results from exposure to chemical and biological agents
• Acquired immunodeficiency syndrome (AIDS) is caused by a virus
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Acquired Immune System Evasion by Pathogens
• Pathogens have evolved mechanisms to attack immune responses
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Antigenic Variation
• Through antigenic variation, some pathogens are able to change epitope expression and prevent recognition
• The human influenza virus mutates rapidly, and new flu vaccines must be made each year
• Human viruses occasionally exchange genes with the viruses of domesticated animals
• This poses a danger as human immune systems are unable to recognize the new viral strain
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Fig. 43-25
Weeks after infection
Mill
ions
of p
aras
ites
per m
L of
blo
od
Antibodies to variant 1 appear
Antibodies to variant 2 appear
Antibodies to variant 3 appear
Variant 3 Variant 2 Variant 1
25 26 27 28 0
0.5
1.0
1.5
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Latency
• Some viruses may remain in a host in an inactive state called latency
• Herpes simplex viruses can be present in a human host without causing symptoms
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Attack on the Immune System: HIV
• Human immunodeficiency virus (HIV) infects helper T cells
• The loss of helper T cells impairs both the humoral and cell-mediated immune responses and leads to AIDS
• HIV eludes the immune system because of antigenic variation and an ability to remain latent while integrated into host DNA
Animation: HIV Reproductive Cycle
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Fig. 43-26
Latency
Relative antibody concentration
AIDS H
elpe
r T c
ell c
once
ntra
tion
in b
lood
(cel
ls/m
m3 )
Helper T cell concentration
Relative HIV concentration
Years after untreated infection 0 1 2 3 4 5 6 7 8 9 10
0
200
400
600
800
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• People with AIDS are highly susceptible to opportunistic infections and cancers that take advantage of an immune system in collapse
• The spread of HIV is a worldwide problem
• The best approach for slowing this spread is education about practices that transmit the virus
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Cancer and Immunity
• The frequency of certain cancers increases when the immune response is impaired
• Two suggested explanations are
– Immune system normally suppresses cancerous cells
– Increased inflammation increases the risk of cancer
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Fig. 43-UN1 Stem cell
Cell division and gene rearrangement
Antigen
Clonal selection
Elimination of self-reactive B cells
Formation of activated cell populations Antibody
Microbe
Memory cells Effector B cells
Receptors bind to antigens
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Fig. 43-UN2
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You should now be able to:
1. Distinguish between innate and acquired immunity
2. Name and describe four types of phagocytic cells
3. Describe the inflammation response
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4. Distinguish between the following pairs of terms: antigens and antibodies; antigen and epitope; B lymphocytes and T lymphocytes; antibodies and B cell receptors; primary and secondary immune responses; humoral and cell-mediated response; active and passive immunity
5. Explain how B lymphocytes and T lymphocytes recognize specific antigens
6. Explain why the antigen receptors of lymphocytes are tested for self-reactivity
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7. Describe clonal selection and distinguish between effector cells and memory cells
8. Describe the cellular basis for immunological memory
9. Explain how a single antigen can provoke a robust humoral response
10. Compare the processes of neutralization and opsonization
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11. Describe the role of MHC in the rejection of tissue transplants
12. Describe an allergic reaction, including the roles of IgE, mast cells, and histamine
13. Describe some of the mechanisms that pathogens have evolved to thwart the immune response of their hosts
14. List strategies that can reduce the risk of HIV transmission