Download 8 Immunology

Immunology
Chapter 8
Immunity and the Immune Response
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Innate immunity
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Molecules that pre-exist in the body that recognize
common microbial motifs
Adaptive Immunity
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Passive
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Administration of antibodies specific to a pathogen
Antibodies neutralize pathogen before disease
onset
Active
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T and B lymphocytes recognize pathogen and
specifically target it for destruction
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Mechanisms of Innate Immunity
Physiologic barriers
Skin
Mucous membranes
Innate immunologic mechanisms
Organs and tissues involved in recognizing foreign substances
Phagocytic cells are strategically located in these organs,
adjacent to blood and lymphatic vessels
Cells
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Neutrophils - phagocytic and possess inflammatory granules (e.g.,
histamine)
Macrophages - phagocytic cells that exist in nearly all tissues that monitor
for infectious agents
Inflammation
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Mediated by cytokines, which are small soluble proteins secreted by many
cells
Fever - caused by interleukin-1 that acts upon hypothalamus
Interferons
Natural killer cells - cause lots of collateral damage
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Mechanisms of Specific Host Defense
Adaptive response
Antibody (humoral) response
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Now referred to as Type 2 immunity
Prominent antibody response
But cells are involved, too!
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Cell-Mediated response
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Now referred to as Type 1 immunity
Prominent cellular response
But antibodies are involved, too!
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Cells are lymphocytes
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B cells
Restricted to lymphoid tissues (e.g., lymph nodes)
Secrete antibodies specific for the pathogen
T cells - circulate through blood and lymph
Helper T cells (Th)
Direct the actions of other cells by secreting cytokines that
signal and coordinate such activities
Cytotoxic T cells (CTL)
Recognize cells infected by viruses and kills them
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Mechanisms of Specific Host Defense
Antigens (Ag)
Any substance that can elicit an immune response in an animal
The body can distinguish self molecules from nonself molecules
Failure of this system can result in autoimmune diseases
Most antigens are large proteins
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Some carbohydrates can be antigenic, but they are generally poor antigens
because T cells cannot recognize them
Antigenic determinants (epitopes) are the parts of an antigen that
are recognized by lymphocytes
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B cells recognize parts of the 3D structure of an antigen
T cells recognize linear peptides of 8-14 amino acids of a protein antigen
Genetic constitution of the host contributes to disease susceptibility
Dosage, route and time of antigen administration influence immune
responses
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Adjuvants are usually required to elicit an effective immune response to nonliving antigens
These are usually irritants that induces localized inflammation
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Mechanisms of Specific Host Defense
Cellular basis of the immune response
B cells
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Develop from stem cells in the bone marrow
The B cell repertoire from the development results in billions of B cells specific
for different antigenic determinants
The express an antigen-specific B cell receptor (BCR), which is simply a
membrane-bound antibody
T cells
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Early development is in the bone marrow
Final development is in the thymus and results in billions of T cells specific for
different antigenic determinants
Possess an antigen-specific T cell receptor (TCR) that has substantial
sequence similarity to antibodies
Two types of T cells
Helper T cells (Th)
Respond to extracellular protein antigens
Secrete cytokines that mediate local immune responses
Cytotoxic T cells (CTL)
Respond to intracellular protein antigens
Kill cells that are infected
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Antibodies
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Antibodies (Ab) are immunoglobulins that recognize antigens
All antibodies are immunoglobulins, but not all immunoglobulins are antibodies
They are proteins
B cells express BCR specific for an antigen
Each B cell possesses thousands of identical BCRs on their surfaces
When the antigen enters the body, it must find the few B cells that possess a
BCR capable of binding to it
This can take several days
When recognition occurs, the B cell, with the help of Th cell cytokines, begins to
secrete antibodies in soluble form
The B cell also undergoes clonal expansion; repeated rounds of cell division
The process includes mutations in the antibody genes of the daughter cells
that leads to antibodies with greater affinity for antigen; termed affinity
maturation
The antibodies also switch to other classes of Ig, each with distinct
biological activities
A plasma cell is a B cell that is committed to secreting antibodies for 2 or 3 days,
then it dies
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Antibodies
Antibodies
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Antibody structure
Composed of two identical heavy chains
and two identical light chains
Form a disulfide-linked tetramers
Each antibody molecule includes:
Two variable regions that interact with
the antigen
Some antibodies bind to viruses in
such a way as to interfere their
infectious capacity
These are termed neutralizing
antibodies
A constant region that confers
biological activities, such as
Binding to receptors on phagocytic
cells
Binding to complement proteins
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Antibodies
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Immunoglobulin classes
IgM
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First to be made during a primary (initial) response
Often secreted as a pentamer of 5 IgM antibodies covalently-linked to one
another (Fig 8-4)
Low affinity, but high avidity
IgG
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Most common serum antibody
Occurs after the IgM response
High affinity
Persist for years in the blood
Four subclasses encoded by different genes
IgG1
IgG2
IgG3
IgG4
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Antibodies
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Immunoglobulin classes (cont.)
IgA
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Most common antibody produced by the body
Secreted into mucosal areas
Often found as a dimer of two covalently-linked IgA molecules
IgE
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Associated with parasitic and helminth infections, and with
hypersensitivities
IgD
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Expressed as a BCR only
Immunoglobulin class switching and affinity maturation
All secreted antibodies are initially IgM
As the immune response ensues, the class switches to IgG, IgA or IgE
This phenomenon is also accompanied by affinity maturation
Both of these events require T cell help
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Cell Surface Receptors for Antigen
B cell receptor for antigen
Naive B cells express IgM or IgD as their surface receptor
(sIg)
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The antibody receptor possesses transmembrane domains at the Cterminus of the H chains
This domain anchors the H-chain in the membrane of the rough ER when
the polypeptides are synthesized
Thus, the antibody receptor protrudes from the B cell’s plasma membrane
This receptor is usually of low affinity because it’s only a close-match to its
antigenic epitope (10-7 KD)
This receptor is used to capture the antigen, which is then internalized by
receptor-mediated endocytosis
When a B cell is activated (i.e., immune stimulus), transcriptional
processing of the antibody mRNA results in a transcript free of the codons
for the transmembrane domain, thus the antibody is secreted in a vesicle
Memory B cells have IgG, IgA or IgE sIg
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They have undergone class switching
These receptors are high affinity because they have undergone affinity
maturation (up to 10-11 KD)
They are extremely efficient at capturing antigen
All B cells have a sIg-associated Igα/Igβ signal transduction
Cell Surface Receptors for Antigen
Cell Surface Receptors for Antigen
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T cell receptor
Transmembrane heterodimer (not secreted)
Structure is similar to the V/H domain of an antibody
Two types (only one type found on a given T cell)
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γδ (gamma-delta)
Two cells
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αβ (alpha-beta)
Helper T cells (Th) express CD4 on their surfaces
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CD4 interacts with MHC class II proteins on antigen presenting cells
(APC)
Cytotoxic T cells (CTL) express CD8 on their surfaces
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CD8 interacts with MHC class I proteins on APC
All T cells have CD3 and ζ-chain (zeta-chain), signal
transduction components
Cell Surface Receptors for Antigen
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Surface Receptors
The Cell
Major Histocompatibility
Complex
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for Antigen
Large contiguous region of mammalian genomes that encode
about 100 proteins involved in antigen processing and presentation
Name is a misnomer; it was first characterized as controlling tissue
rejection in mice in the 1930s-1950s
Two principal classes
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Class I molecules (3 loci in humans)
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Expressed by all nucleated cells, which act as APC
Responsible for presenting peptides from endogenously-synthesized
polypeptides (i.e., made on cellular ribosomes)
At rough ER, viral polypeptides are clipped into 8-10 amino acid
peptides by the proteosome and then transported into the lumen of the
ER
Class I molecules bind viral peptides at the ER, then traffic to the cell
surface
Marks the cell for death by CTL (via CD8 interaction with class I
molecule)
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Cell Surface Receptors for Antigen
The Major Histocompatibility Complex (cont.)
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Two principal classes (cont.)
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Class II molecules (3 loci in humans)
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Expressed by professional APC
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B cells
Macrophages
Dendritic cells
A few others
For antigens captured by receptor-mediated endocytosis or
phagocytosis (i.e., exogenous)
Fusion of phagosome with class II-bearing endosome results
phagolysosome
Peptides bind to class II proteins
The class II molecules traffic to the cell surface where they are
engaged by helper T cells
CD4 binds to the class II molecule, thus restricting the interaction
to Th cells
Cell Surface Receptors for Antigen
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exogenous (extracellular) antigens
Type IIfor(Antibody-Mediated)
Immunity
• The primary response
• Within days after infection, antigen-specific IgM appears
• Within a week, antigen-specific IgG appears
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As antigen becomes limiting (i.e., the immune system clears
the infection) the immune response wanes
Most (98%) responding B cells die, but a few remain as
memory cells
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Class switching and affinity maturation improve the efficiency of the
antibody response
Unlike naive B cells with IgM, memory cells possess high-affinity sIgG or
sIgA
The secondary (anamnestic) response
In the event the pathogen invades again, the immune
response will be of greater speed and magnitude because of
the memory cells
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No need for class switching
No need for affinity maturation
Antibody response is quicker and of higher affinity
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Type II (Antibody-Mediated) Immunity
The role of helper T cells in Type II immunity
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Exogenous antigens (such as free virus) are captured by professional
APC
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The antigen is imported into a lysosome (phagolysosome) that has
more than 60 types of molecules, including digestive enzymes
Proteins are digested into peptides of varying length
The phagolysosome is fused with another endosome containing
nascent MHC class II proteins
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B cell uses its sIg receptor
Macrophages are phagocytic
Dendritic cells are pinocytotic
Packaged with the class II endosome is the invarient chain, which occludes the
peptide binding cleft to prevent premature peptide binding
The peptides bind to the class II proteins and additional protease
activity occurs until the peptides are about 11-15 amino acids
The vesicle traffics to the cell surface where it fuses with the plasma
membrane (i.e., class II/peptide complex is outside of the cell)
The Th cell must recognize both the class II protein and peptide before
B cell
Helper
T cell
Cytokines that modulate the immune
response (e.g., class switching,
affinity maturation, antibody production)
Secreted
antibodies
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Type I (Cell Mediated) Immunity
Principally for containment of endogenously-produced (intracellular)
antigens
Viruses rely upon cellular ribosomes for the biosynthesis of their
polypeptides
A small proportion of all polypeptides (self and non-self) synthesized in
a cell are re-directed to the proteosome complex in the cytoplasm
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The proteosome is a tubular protein structure that has protease activity
Polypeptides enter the proteosome, which can hold about 8-10 amino acids, then its
protease activity results in clipping into peptides
The peptides are ejected from the other end of the proteosome
The transporter associated with antigen processing (TAP) translocates
the peptides into the lumen of the ER
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This event requires ATP
Inside the ER, nascent MHC class I molecules bind the peptides
After budding from the ER, the class I/peptide traffic to the Golgi, then
the cell surface
The class I/peptide complex is displayed on the outside of the cells for
scanning by the CTL repertoire
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The CTL kills the cell by sending it a death signal (apoptosis) or by secreting
The Complement System
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The complement system is composed of several pre-existing
blood proteins that puncture membranes
There are three pathways
The classical pathway is antibody-dependent
The MB lectin pathway
The alternative pathway
This cascade is a series of enzymatic reactions carried out by
the complement proteins
Two final outcomes occur
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Complement proteins attack the envelope, thus compromising it
The complement-bound viruses are opsonized, that is, they are more
efficiently phagocytosed
The Complement System
Cytokines
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Cytokines are hormone-like proteins
There are more than 80 known
Interleukins - secreted by leukocytes
Interferons - potent antivirals
Chemokines - mediate chemotaxis
Inflammation/immunosuppression
Potent, often active at picomolar concentrations
Bind to receptors on target cells and modulate gene expression
Some are used therapeutically (recombinant)
Some are associated with T cell subsets
Th1 cells secrete...
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IFNγ
TNF
LT
Th2 cells secrete...
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IL-4
IL-5
Overly Aggressive Immune
Responses
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Often results in immunopathology
There are no good clinical strategies for managing
pathologic immune responses
Immunologic Diagnostic Tests
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The use of blood to detect antigen-specific antibodies in
testing is termed serology
Relies upon the fact that serum antibodies to specific
pathogens cannot be detected until after infection (or
immunization)
Provides diagnostic indicator of infection
IgM - current/recent infection
IgG - current or past infection (decades)
Common serological tests
Enzyme-linked immunosorbant assay (ELISA)
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Rapid
High throughput
Inexpensive
Immunofluorescent antibody (IFA)
Immunoblotting
Immunologic Diagnostic Tests
Immunologic Diagnostic Tests
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Immunoblotting
More specific and sensitive than ELISA
Also more work and more expensive
Most common is western blot
First step is electrophoresis of antigen (SDS-PAGE)
Second is transfer of antigen from gel to solid phase, such as
nitrocellulose membrane
Steps for western blot
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Block membrane with irrelevant protein (nonfat powdered milk solution)
Incubate serum sample
Incubate anti-antibody, enzyme conjugate
Incubate substrate
Look for bands appearing on membrane
Immunologic Diagnostic Tests