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Alveolar (Lung) Macrophage Attacking E. coli (SEM
x10,000) © Dr Dennis Kunkel (used with permission)
Macrophage Attacking E.coli (SEM x8,800) © Dr Dennis Kunkel
I. Chain of Infection
Horton & Parker: Informed Infection Control Practice
HOST DEFENSE
MECHANISMS
Lecture nr. 6
Antigens. Antibodies.
Immune response.
Cell cooperation and
mediators in immune
response.
15.03.2011
HOST DEFENSE
MECHANISMS
Definitions of the
Immunity
Immunity: “Free from burden”. Ability of an
organism to recognize and defend itself against
specific pathogens or antigens.
Immunity refers to the ability of the human body to
resist disease agents and their toxins through
possession of antibodies.
Immunity is a long lasting protection against a
specific antigen.
Specific Host Defenses:
The Immune Response
T-cell (part of the specific
immune response)
The structure of immunoglobulins
The Immune Response
Immune Response: Third line of defense.
Involves production of antibodies and
generation of specialized lymphocytes
against specific antigens.
What is the immune
system?
 The body’s defense against disease causing
organisms, malfunctioning cells, and foreign
particles
How does the immune system
respond to different infections?

Microbes are recognized by two mechanisms:



evolved broad recognition mechanisms (innate
immunity), and
by highly specific lymphocyte antibodies and T cell
receptors (adaptive immunity)
Different types of microbes are eliminated by
different effectors mechanisms, which are
designed to best combat each type of microbe
Classification
of immunity
Characteristics of Innate
and Adaptive Immunity
Innate Immunity
Acquired Immunity
Antigen independent
Antigen dependent
Immediate
Take time
Genetically determined
No Immunologic
memory
Not genetically determined.
Development
of memory
Classification
of the immunity
Acquired Immunity:
Obtained in the course of daily
life.
A. Naturally Acquired Active Immunity:



Antigens or pathogens enter body naturally.
Body generates an immune response to antigens.
Immunity may be lifelong (chickenpox or mumps) or temporary
(influenza or intestinal infections).
B. Naturally Acquired Passive Immunity:




Antibodies pass from mother to fetus via placenta or breast feeding
(colostrum).
No immune response to antigens.
Immunity is usually short-lived (weeks to months).
Protection until child’s immune system develops.
Classification
of the immunity
II. Artificially Acquired
Immunity: Obtained by receiving a
vaccine or immune serum.
1. Artificially Acquired Active Immunity:
 Antigens are introduced in vaccines (immunization).
 Body generates an immune response to antigens.
 Immunity can be lifelong (oral polio vaccine) or temporary
(tetanus toxoid).
2. Artificially Acquired Passive Immunity:
 Preformed antibodies (antiserum) are introduced into body
by injection.



Snake antivenom injection from horses or rabbits.
Immunity is short lived (half life three weeks).
Host immune system does not respond to antigens.
Significance of the
Immune System


Beneficial:

Protection from Invaders

Elimination of Altered Self
Detrimental:

Discomfort (inflammation)

Damage to self (autoimmunity)
Components of Human
Immune System
Cells of the immune system
 Lymphocytes
 Mediators of adaptive immune responses; only cells with
specific receptors for antigens
 Antigen-presenting cells (APCs)
 Specialized to capture, concentrate, and display antigens for
recognition by lymphocytes
 Dendritic cells; macrophages, B cells; follicular dendritic
cells
 Different APCs serve different roles in adaptive immune
responses
 Effector cells
 Function to eliminate microbes; include lymphocytes,
granulocytes (neutrophils, eosinophils), macrophages
Lymphocytes
Originate :
in liver, spleen and bone marrow of fetus
in bone marrow after birth
From stem cells – hemocytoblasts – that
produce all blood cells.
Lymphocytes
To become mature, immunocompetent cells, they must
pass through lymphoid tissues in other parts of the
body.
As they do so, they become committed to
becoming either T cells or B cells
1. Cells that migrate through the bone marrow
(bursal equivalent) become B cells, and will produce
antibodies and participate in humoral immunity.
2. Cells that migrate through the thymus glands
become T cells and participate in Cell-mediated
immunity.
LYMPHOCYTE DEVELOPMENT
Congenital immunodeficiency diseases are often caused by blocks
at different stages of lymphocyte maturation
CD Nomenclature
•
•
Structurally defined leukocyte surface molecule that
is expressed on cells of a particular lineage
(“differentiation”) and recognized by a group
(“cluster”) of monoclonal antibodies is called a
member of a cluster of differentiation (CD)
CD molecules (CD antigens, CD markers) are:
•
•
•
•
Identified by numbers
Used to classify leukocytes into functionally distinct
subpopulations, e.g. helper T cells are CD4+CD8-, CTLs are
CD8+CD4Often involved in leukocyte functions
Antibodies against various CD molecules are used to:
•
•
•
Identify and isolate leukocyte subpopulations
Study functions of leukocytes
Eliminate particular cell populations
Two types of T cells
CD4 T cells
Help other immune cells
Recognize peptide + MHC
II
CD8 T cells
Kill virus-infected cells
Recognize peptide + MHC
I
MHC II is expressed
primarily on immune
cells
MHC I is expressed on
all nucleated cells
Peptides are from
endocytosed antigen
Peptides are from
cytosolic antigen
ANTIGENS
Antigens
 An antigen is a molecule that stimulates an immune
response.
 The word originated from the notion that they can
stimulate antibody generation.
 The modern definition includes all substances that can be
recognized by the adaptive immune system.
Antigens
•Antigens - substances recognized as
“non-self”. These can be:
Infectious agents - bacteria, viruses, fungi or
parasites
Noninfectious substances –
• Environmental - pollen, foods, bee, venoms
• Drugs, vaccines, transfusions and transplanted
tissues
Chemical nature of
antigen




Proteins
Polysaccharides
Nucleic acids
Lipids

Some glycolipids and phospholipids can be
immunogenic for T cells and illicit a cell-mediated
immune response
Antigens
 Most are proteins or large polysaccharides from a
foreign organism.


Microbes: Capsules, cell walls, toxins, viral capsids, flagella,
etc.
Nonmicrobes: Pollen, egg white , red blood cell surface
molecules, serum proteins, and surface molecules from
transplanted tissue.
 Lipids and nucleic acids are only antigenic when combined
with proteins or polysaccharides.
Antigens
Antibody Generator
The best antigens are:
1.
2.
3.
large
recognized as foreign
complex
Antigens can be classified in
order of their origins
 Exogenous antigens - are antigens that have
entered the body from the outside,


ex. by inhalation, ingestion, or injection.
By endocytosis or phagocytosis, these antigens are taken into
the antigen-presenting cells (APCs) and processed into
fragments.
 Endogenous antigens - antigens that have
been generated within the cell, as a result of:


normal cell metabolism, or
viral or intracellular bacterial infection.
Types of Antigens
Allogenic antigen
The specific antigen exists in different individuals.
Blood type antigens.
Autoantigens
usually a normal protein (sometimes DNA or RNA)
that is recognized by the immune system of patients
suffering from a specific autoimmune disease.
Tumor antigens
 are presented by the MHC I molecules on the surface of
tumor cells.
 can sometimes be presented only by tumor cells and never
by the normal cells.
 they are called tumor-specific antigens (TSAs) and typically
result from a tumor specific mutation.
Characteristics of
Antigen
Immunogenicity
The capacity to stimulate the production of
antibodies or cell-mediated immune
responses.
Characteristics of
Antigen
Antigenicity
The ability to bind antibody.

Complete antigen

Incomplete antigen, also known as hapten.
Hapten: Small foreign molecule that is not antigenic. Must be
coupled to a carrier molecule to be antigenic. Once antibodies
are formed they will recognize hapten.
Antigens
Epitope or, Antigenic determinants:
Small part of an antigen that interacts with an
antibody.
Any given antigen may have several epitopes.
Each epitope is recognized by a different
antibody and may interact with them by
paratopes of Antibody
Epitopes: Antigen Regions that Interact
with paratopes, Antibodies regions
Types of Epitopes
1. Linear epitopes
 continuous and found in polysaccharides as well
as in both native (nondenatured) and denatured
proteins, especially fibrillar proteins.
 specificity depends upon primary sequence.
 typical size is 5-6 subunits in length.
Types of Epitopes
2. Conformational epitopes
 Discontinuous involve multiple subunits,
often located far apart in the primary
sequence of the antigen molecule and are
thus found only in native (globular) proteins.
Antigenic epitopes
ANTIBODY
Antibodies
 Proteins that recognize and bind to a particular antigen with
very high specificity.
 Made in response to exposure to the antigen.
 One virus or microbe may have several antigenic determinant
sites, to which different antibodies may bind.
 Each antibody has at least two identical sites that bind antigen:
antigen binding sites.
 Valence of an antibody: Number of antigen binding sites.
Most are bivalent.
 Belong to a group of serum proteins called immunoglobulins
(Igs).
Antibody Structure
 Monomer: A flexible Y-shaped molecule with four
protein chains:
 2 identical light chains
 2 identical heavy chains
 Variable Regions: Two sections at the end of Y’s
arms. Contain the antigen binding sites (Fab).
Identical on the same antibody, but vary from one
antibody to another.
 Constant Regions: Stem of monomer and lower parts
of Y arms.
 Fc region: Stem of monomer only. Important
because they can bind to complement or cells.
Immunoglobulins
 IgG - monomer
 IgA – dimer – 2 units
 IgM – pentamer – 5 units
 IgD – monomer
 IgE – monomer
I. IgG
 Structure: Monomer
 Percentage serum antibodies: 80%
 Location: Blood, lymph, intestine
 Half-life in serum: 23 days
 Complement Fixation: Yes
 Placental Transfer: Yes
 Known Functions: Enhances phagocytosis, neutralizes
toxins and viruses, protects fetus and newborn.
II. IgM
 Structure: Pentamer
 Percentage serum antibodies: 5-10%
 Location: Blood, lymph, B cell surface (monomer)
 Half-life in serum: 5 days
 Complement Fixation: Yes
 Placental Transfer: No
 Known Functions: First antibodies produced during an
infection. Effective against microbes and agglutinating
antigens.
III. IgA
 Structure: Dimer
 Percentage serum antibodies: 10-15%
 Location: Secretions (tears, saliva, intestine, milk), blood
and lymph.
 Half-life in serum: 6 days
 Complement Fixation: No
 Placental Transfer: No
 Known Functions: Localized protection of mucosal surfaces
Provides immunity to infant digestive tract.
IV. IgD
 Structure: Monomer
 Percentage serum antibodies: 0.2%
 Location: B-cell surface, blood, and lymph
 Half-life in serum: 3 days
 Complement Fixation: No
 Placental Transfer: No
 Known Functions: In serum function is unknown. On B cel
surface, initiate immune response.
V. IgE
 Structure: Monomer
 Percentage serum antibodies: 0.002%
 Location: Bound to mast cells and basophils throughout
body. Blood.
 Half-life in serum: 2 days
 Complement Fixation: No
 Placental Transfer: No
 Known Functions: Allergic reactions. Possibly lysis of
worms.
Clonal Selection of B Cells is Caused by
Antigenic Stimulation
Secretions the
Antibodies by B cells
B cells develop from stem cells in the bone marrow
of adults (liver of fetuses).

After maturation B cells migrate to lymphoid organs
(lymph node or spleen).

Clonal Selection: When a B cell encounters an antigen it
recognizes, it is stimulated and divides into many clones
called plasma cells, which actively secrete antibodies.

Each B cell produces antibodies that will recognize only
one antigenic determinant.
Apoptosis
Programmed cell death (“Falling away”).

Human body makes 100 million lymphocytes
every day. If an equivalent number doesn’t die,
will develop leukemia.

B cells that do not encounter stimulating antigen
will self-destruct and send signals to phagocytes
to dispose of their remains.

Many virus infected cells will undergo apoptosis,
to help prevent spread of the infection.
Antigen presenting
cells
Antigens must be processed in
order
to be recognised by T cells
T
Y
Soluble
native Ag
Cell surface
native Ag
Soluble
peptides
of Ag
Cell surface peptides of
Ag presented by cells that
express MHC antigens
Cell surface
peptides
of Ag
ANTIGEN
PROCESSING
No T cell
response
No T cell
response
No T cell
response
No T cell
response
T cell
response
The interaction of T cells with macrophages
requires antigen catabolism
Specific
T cells
T
Microbes NO T CELLS NO T CELLS NO T CELLS
BIND
BIND
BIND
M
0mins
M
M
T CELLS
BIND
M
60mins
T cell do not bind stably to antigen presenting cells unless the
antigen is catabolised
Summary
• T and B cells recognise antigen differently
• Antigen must be catabolised before T cells can recognise it
• Antigen processing generates antigenic peptides
• Exogenous antigen processing takes place in lysosomes
• Endogenous processing is non-lysosomal
• The mechanism of antigen processing depends upon the
compartment in which the pathogen replicates
• Endogenous and exogenous antigen processing both involve
uptake, degradation, complex formation and presentation
• Pathogens can evade immunity by disrupting antigen processing
Immune response
Duality of Immune
System
I.
Humoral (AntibodyMediated) Immunity
I.
II. Cellular (Cell
Mediated) Immunity
I. Humoral (AntibodyMediated) Immunity






Involves production of antibodies against foreign antigens.
Antibodies are produced by a subset of lymphocytes called B
cells.
B cells that are stimulated will actively secrete antibodies and
are called plasma cells.
Antibodies are found in extracellular fluids (blood plasma,
lymph, mucus, etc.) and the surface of B cells.
Defense against bacteria, bacterial toxins, and viruses that
circulate freely in body fluids, before they enter cells.
Also cause certain reactions against transplanted tissue.
How do antibodies actually
help eliminate antigens?
Consequences of
Antigen-Antibody Binding
Antigen-Antibody Complex: Formed when
an Ab binds to Ag.
Affinity: A measure of binding strength.
1. Agglutination: Antibodies cause antigens
(microbes) to clump together.
 IgM is more effective that IgG.

Hemagglutination: Agglutination of red blood cells.
Used to determine ABO blood types and to detect
influenza and measles viruses.
Consequences of
Antigen-Antibody Binding
2. Opsonization: Antigen (microbe) is covered
with antibodies that enhances its ingestion
and lysis by phagocytic cells.
3. Neutralization: IgG inactivates viruses by
binding to their surface and neutralize toxins
by blocking their active sites.
Consequences of
Antigen-Antibody Binding
4.
Antibody-dependent cell-mediated cytotoxicity:
Used to destroy large organisms (e.g.: worms).
Target organism is coated with antibodies and
bombarded with chemicals from nonspecific
immune cells.
5. Complement Activation: Both IgG and IgM
trigger the complement system which results in
cell lysis and inflammation.
Consequences of Antibody Binding
Immunological responce
Primary Response:
 After initial exposure to antigen, no antibodies are
found in serum for several days.
 A gradual increase in titer, first of IgM and then
of IgG is observed.
 Most B cells become plasma cells, but some B cells
become long living memory cells.
 Gradual decline of antibodies.
Immunological responce
Secondary Response:
Subsequent exposure to the same antigen displays a
faster and more intense antibody response.

Increased antibody response is due to the existence of
memory cells, which rapidly produce plasma cells upon
antigen stimulation.
Antibody Titer: The amount of antibody in the serum.
Antibody Response After Exposure to
Antigen
Duality of Immune
System
I.
Humoral (AntibodyMediated) Immunity
I.
II. Cellular (Cell
Mediated) Immunity
II. Cellular (Cell
Mediated) Immunity

Involves specialized set of lymphocytes called T cells
that recognize foreign antigens on the surface of cells,
organisms, or tissues:


Helper T cells
Cytotoxic T cells

T cells regulate proliferation and activity of other cells
of the immune system: B cells, macrophages,
neutrophils, etc.

Defense against:




Bacteria and viruses that are inside host cells and are inaccessible to
antibodies.
Fungi, protozoa, and helminths
Cancer cells
Transplanted tissue
T Cells and Cell Mediated
Immunity
Antigens that stimulate this response are mainly
intracellular.
Requires constant presence of antigen to remain effective.
Unlike humoral immunity, cell mediated immunity is not
transferred to the fetus.
Cytokines: Chemical messengers of immune cells.
 Over 100 have been identified.
 Stimulate and/or regulate immune responses.



Interleukins: Communication between WBCs.
Interferons: Protect against viral infections.
Chemokines: Attract WBCs to infected areas.
T Cells and Cell Mediated
Immunity

T cells are key cellular component of immunity.

T cells have an antigen receptor that recognizes
and reacts to a specific antigen (T cell receptor).

T cell receptor only recognize antigens
combined with major histocompatability
(MHC) proteins on the surface of cells.
MHC Class I: Found on all cells.
 MHC Class II: Found on phagocytes.

T Cells Only Recognize Antigen
Associated with MHC Molecules on Cell
Surfaces
Cells and Cell Mediated
Immunity
Types of T cells
1. T Helper (TH) Cells: Central role in immune
response.
Most are CD4+
 Recognize antigen on the surface of antigen presenting
cells (e.g.: macrophage).
 Activate macrophages
 Induce formation of cytotoxic T cells
 Stimulate B cells to produce antibodies.

Central Role of Helper T Cells
Types of T cells
(Continued)
2. Cytotoxic T (Tc) Cells: Destroy target
cells.
Most are CD4 negative (CD4 -).
 Recognize antigens on the surface of all cells:

 Kill host cells that are infected with viruses or bacteria.
 Recognize and kill cancer cells.
 Recognize and destroy transplanted tissue.
Release protein called perforin which forms a pore in
target cell, causing lysis of infected cells.
 Undergo apoptosis when stimulating antigen is gone.

Cytotoxic T Cells Lyse Infected
Cells
Types of T cells
(Continued)
3. Delayed Hypersensitivity T (TD) Cells: Mostly T
helper and a few cytotoxic T cells that are involved
in some allergic reactions (poison) and rejection of
transplanted tissue.
4. T Suppressor (Ts) Cells: May shut down
immune response.
Nonspecific Cellular
Components
1. Activated Macrophages: Stimulated
phagocytes.



Stimulated by ingestion of antigen
Larger and more effective phagocytes.
Enhanced ability to eliminate intracellular bacteria, virus-infected
and cancerous cells.
2. Natural Killer (NK) Cells:



Lymphocytes that destroy virus infected and tumor cells.
Not specific. Don’t require antigen stimulation.
Not phagocytic, but must contact cell in order to lyse it.
B and T cells mediate
specific immunity
B cells
Matures in
Type of immunity
Secretes
Antigen receptor
Where found
bone marrow
humoral
antibodies
surface Ig
spleen
Targets
bacteria,
viruses
Yes
Memory
T cells
thymus
cell-mediated
cytokines
T cell receptor
blood, lymph
nodes
infected cells,
tumor cells?
Yes
Cellular Interactions in
the Immune Response
Antigen-Presenting cells (macrophages)
place antigen on their cell surface in
combination with the MHC II complex
Ag is presented to a specific helper T cell
that has receptors that bind the Ag – MHC II
complex
After binding, the APC produces Interleukin
-1 (IL-1) which stimulates the TH Cell to
produce IL-2 and/or IL-4
Interleukin-2 has an autocrine function,
causes TH Cell to clone itself, and make
more IL-2 and /or IL-4
Central Role of Helper T Cells
Helper T cells
 TH1 cells produce IL -2 and IFN-ɣ and
influence cell-mediated immunity
 TH2 cells produce IL -4 (and other IL’s) and
influence antibody-mediated (humoral)
immunity
When B cell comes in contact with the
antigen and IL-4, the B cell produces plasma
cells and memory cells
Tc Cells come in contact with the antigen on
the surface of infected cells in combination
with the MHC 1 complex. When also have
binding with IL-2, cells produce activated Tc
Cells and memory cells.
Relationship Between CellMediated and Humoral
Immunity
Antibody Production
T-Dependent Antigens:




Antibody production requires assistance from T helper cells.
A macrophage cells ingest antigen and presents it to TH cell.
TH cell stimulates B cells specific for antigen to become plasma cells.
Antigens are mainly proteins on viruses, bacteria, foreign red blood cells,
and hapten-carrier molecules.
T-Independent Antigens:



Antibody production does not require assistance from T cells.
Antigens are mainly polysaccharides or lipopolysaccharides with
repeating subunits (bacterial capsules).
Weaker immune response than for T-dependent antigens.
Humoral Response to T Dependent
Antigens
Overview of the Immune Response
Principal mechanisms of defense against microbes
Antibodies
Phagocytes
T cells (CTLs)
(may work with antibodies, T cells)
All microbes
All microbes
Intracellular
microbes, esp.
viruses
Disorders of Immunity
Hypersensitivity
Exaggerated Immune Response
Hypersensitivity Types
 Allergy

Exogenous, non-human antigen
 Isoimmunity (alloimmunity)

Exogenous, human antigen
 Autoimmunity

Endogenous antigen
Hypersensitivity
Mechanisms




Type I: IgE mediated
Type II: Tissue specific
Type III: Immune complex mediated
Type IV: Cell mediated
Type I
 Immediate hypersensitivity
 IgE mediated
 Exogenous antigen
Most (but not all) Allergies
Type I: Mechanism
 Repeated antigen exposure causes increased IgE
production
 IgE binds to mast cells
 Sensitization occurs
Type I: Mechanism
 Antigen binds to IgE on mast cell membrane
 Mast cell releases histamine, chemotaxic factors
 Inflammatory response occurs
Type I: Signs/Symptoms
 Clinical signs, symptoms = response to histamine
release
 GI, skin, respiratory system


High mast cells numbers
Most sensitive
Type I: Signs/Symptoms
 Histamine effects



Vasodilatation
Increased capillary permeability
Non-vascular smooth muscle spasm
Type I: Signs/Symptoms
 Skin: flushing, itching, edema, urticaria
 Respiratory: bronchospasm, laryngospasm, laryngeal
edema
 Cardiovascular: tachycardia, hypotension
 GI: nausea, vomiting, cramping, diarrhea
Type I: Atopia
 “Allergy prone” individuals
 Genetic predisposition
 More IgE
 More mast cell receptors for antibodies than
normal
Type I: Anaphylaxis
 Severe, generalized Type I reaction
 Life-threatening



Loss of airway
Ventilatory failure
Hypoperfusion
Type II




Tissue specific
Reaction to tissue-specific antigens
Causes target cell destruction, dysfunction
Exogenous or endogenous antigen
Type II
 Most commonly affected cells


Red blood cells
Thyroid cells
Type II: Mechanisms
 Antibody binds to cell membrane, triggers
compliment-mediated lysis
 Examples


Reaction to transfused blood
Hemolytic disease of newborn
Type II: Mechanisms
 Antibodies promote target cell clearance by
macrophages
Type II: Mechanisms
 Antibodies bind to target cells and cytotoxic
T-cells
 Trigger release of toxins to destroy target
cells
Type II: Mechanisms
 Antibody binds to cell membrane, causes alterations in
target cell function
 Example: Graves' disease




Antibody binds to thyroid cell membrane
Mimics Thyroid Stimulating Hormone action
Causes production of excessive amounts of thyroid hormone
Results in common form of hyperthyroidism
Type III
 Mediated by antigen/ antibody complex
deposition in tissues
 Exogenous or endogenous antigen
Type III: Mechanism
 Ag-Ab complex deposited in tissues
 Especially sensitive tissues are blood vessels, GI,
respiratory system
 Causes complement activation, increased
neutrophil activity
 Neutrophils have trouble digesting complexes,
release lysosomes causing damage
Type III
 Immune complex quantity varies over time
 Symptomatic periods alternate with periods of
remission
Type III: Serum Sickness
 Repeated intravenous antigen injections
 Immune complexes deposited in tissues
 Fever, rash, pain, lymphadenopathy
Type III: Raynaud’s
Phenomenon
 Temperature governs immune complex
deposition in peripheral circulation
 Exposure to cold causes redness, pain of fingers,
toes followed by numbness, cyanosis, gangrene
Type III: Arthus Reaction
 Occurs after repeated LOCAL exposure to
exogenous antigen
 Immune complexes in vessel walls
 Examples


Celiac disease from wheat protein
Hemorrhagic alveolitis from moldy hay inhalation
Type IV
 Delayed
 Mediated by Td (lymphokine-producing) or Tc
(cytotoxic) cells
 No antibody involved
Type IV
 Examples


Graft rejection
Contact allergic reactions (poison ivy)
Hypersensitivity Targets
 Allergins



Pollen (hay fever)
Drug reactions
Foods
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