Normal Immune System
Function
Karim Rafaat
Organs of the Immune System
Central
lymphoid
organs
Central (primary)
lymphoid organs are the
sites for generation and
early maturation of
lymphocytes
T cells mature in the
thymus (T for thymus)
B cells mature in the
bone marrow
B for bursa of Fabricius
(a lymphoid organ in
birds)
[After Hieronymus Fabricius (1537-1619), Italian
anatomist]
Lymphoid organs
contain lymphocytes
and non-lymphoid cells
such as macrophages
and dendritic cells (and
epithelial cells)
Lymphoid organs are
important for the
generation and
maturation of
lymphocytes, the
initiation of immune
responses and the
perpetuation of immune
responses
Peripheral lymphoid
organs
1. trap antigens
2. are the sites for
initiation of most
immune response
3. provide signals for
recirculation of
lymphocytes
4. Antigen delivery to
regions
of increased
*other
stuff
(not traffic
peripheral
lymphoid organs)
Flow of lymph
Gut associated lymphoid tissue (GALT)(tonsils,
adenoids, Peyer’s patches, appendix)
•Immature B and T cells mature in the central lymphoid organs
•Then, they circulate in the blood and through the peripheral
lymphoid organs. As long as they have not encountered the specific
antigen that binds their antigen receptors (BCR or TCR), the
circulating lymphocytes are mature naïve lymphocytes
When they encounter antigen (bind antigen in their antigen receptor),
they
1. Stick in the lymph nodes (or other peripheral lymphoid
organ)(they stop circulating, i.e., altered trafficking)
2. Proliferate (divide)
3. Differentiate
Innate and Specific immunity;
Cells of the Immune System
Cells of the Immune System
Myeloid cells
lymphoid cells
Granulocytic
Monocytic
T-cells
B-cells
Neutrophils
Basophils
Eosinophils
Macrophages
Langerhans &
Kupffer cells
Dendritic cells?
Helper
Cytotoxic
Suppressor
Plasma
cells
Dendritic
cells?
Components of the Immune
System
Nonspecific
Humoral
complement,
interferon,
TNF etc.
Cellular
macrophages,
neutrophils
Specific
Humoral
antibodies
Cellular
T cells; other
effectors cells
Characteristics of Innate and
Adaptive Immunity
Innate Immunity
Adaptive Immunity
Antigen independent
Antigen dependent
No time lag
A lag period
Not antigen specific
Antigen specific
No Immunologic
memory
Development
of memory
Components of Innate and Adaptive
Immunity
Innate Immunity
Adaptive Immunity
physical barriers
skin, gut Villi, lung cilia,etc
soluble factors
many protein and
non-protein secretions
cells
phagocytes, NK cell
eosinophils, K cells
non
e
Immunoglobulins
(antibody)
T and B lymphocytes
Effector mechanisms in Innate
Immunity
Site
Skin
GI tract
Lung
Component
Functions
squamous cells
sweat
desquamation
flushing, fatty acids
columnar
cells
Peristalsis, low pH
bile salts, fatty acids
tracheal cilia
mucociliary elevator
surfactants
Effector mechanisms in Innate
Immunity
Site
Component
Functions
Nasopharynx
and eye
mucus, saliva, tears
flushing, lysozyme
Blood and
Lymphiod
organs
Phagocytes
phagocytosis and
intracellular killing
K, NK & LAK
cells
direct and antibody
dependent cytolysis
Effector mechanisms in Innate
Immunity
Site
Serum and
other serous
fluids
Component
Functions
lactoferrin,
transferrin
iron deprivation
interferons,
TNF-
antiviral proteins
phagocyte activation
lysozyme
Fibronectin &
complement
peptidoglycan hydrolysis
opsonization, enhanced
phagocytosis, inflammation
Phagocytes are the Most Important
Cells
George Bernard Shaw wrote:
“There is at bottom only one
genuine treatment for all
diseases,…to
stimulate
Influenced
by the
work of
phagocytes.theDrugs
are a
Eli Metchnikoff,
delusion. …(when) the
phagocytes are stimulated;
they devour the disease…”
Phagocytes:
Macrophages
 phagocytosis, intracellular
and extra-cellular killing,
tissue repair, antigen
presentation for specific
immune response
 characteristic nucleus and
CD14 membrane marker.
Phagocyte Response to Infection
 The SOS Signals
–N-formyl methionine
–Clotting system
peptides
–Complement products
 Phagocyte response
–Vascular adherence
–Diapedesis
–Chemotaxis
–Activation
–Phagocytosis and killing
Initiation of Phagocytosis
Attachment via
ScavengerR
IgG FcR
CR
Toll-like R
Respiratory Burst
Oxygen Dependent Myeloperoxidase
Independent Reactions
Glucose +NADP+
G-6-P-dehydrogenase
NADPH + O2
Cytochrome b558
Pentose-P
+ NADPH
+
NADP + O2
-
-
2O2 + 2H+
Superoxide dismutase
-
2O2 + H2O2
H2O2 + 1O2
.OH + OH- + 1O2
Mediators of Oxygen Independent
Killing in the Phago-lysosome
Effector Molecule
Function
Cationic proteins (cathepsin)
Damage to microbial
membranes
Lysozyme
Hydrolyses mucopeptides in
the cell wall
Lactoferrin
Deprives pathogens of iron
Hydrolytic enzymes (proteases)
Digests killed organisms
Non-specific Killer Cells
NK and LAK cells
ADCC (K) cell
Activated
macrophages
Eosinophils
They all kill foreign and
altered self targets
Natural Killer (NK) cells
also known as large granular
lymphocytes (LGL)
kill infected and malignant
cells
are identified by the presence
of CD56 & CD16 and
absence of CD3
activated by IL2 and IFN-γ to
become LAK cells
Lymphokine Activated Killer (LAK)
cell
kills
kills
transformed
malignant
and malignant
cells
cells
Innate Immunity
• The complement system
Complement:
history
Discovered in 1894 by
Bordet
It represents lytic activity
of fresh serum
Complement functions
Host benefit:
opsonization to enhance phagocytosis
phagocyte attraction and activation
lysis of bacteria and infected cells
regulation of antibody responses
clearance of immune complexes
clearance of apoptic cells
Host detriment:
Inflammation, anaphylaxis
Proteins of the complement
system (nomenclature)
C1(qrs), C2, C3, C4, C5, C6, C7, C8, C9
factors B, D, H and I, properdin (P)
mannose binding lectin (MBL), MBL associated
serine proteases (MASP-1 MASP-2)
C1 inhibitor (C1-INH, serpin), C4-binding protein
(C4-BP), decay accelerating factor (DAF),
C1 receptor (CR1), protein-S (vitronectin)
Pathways of complement
activation
CLASSICAL
PATHWAY
antibody
dependent
LECTIN
PATHWAY
ALTERNATIVE
PATHWAY
antibody
independent
Activation of C3 and
generation of C5 convertase
activation
of C5
LYTIC ATTACK
PATHWAY
Components of the Classical
Pathway
C3
C1 complex
C4
Classical Pathway
Generation of C3-convertase
Classical Pathway
Generation of C3-convertase
C4b2a is C3 convertase
C4b
Classical Pathway
Generation of C5-convertase
C4b2a3b is C5 convertase;
it leads into the Membrane
Attack Pathway
C4b
C3b
Components of mannose-binding
lectin pathway
MBL
MASP1
Mannose-binding lectin pathway
C4b2a is C3 convertase; it
will lead to the generation of
C5 convertase
MASP1
MBL
Components of the
alternative pathway
C3
Spontaneous C3 activation
Generation of C3 convertase
C3 i
b
C3b
C3iBb complex has a very short half life
C3-activation
the amplification loop
If spontaneously-generated
C3b is not degraded
C3b
b
C3 b
C3-activation
the amplification loop
C3 b
C3b
b
C3b
C3-activation
the amplification loop
C3 b
b
C3b
C3b
C3b
C3-activation
the amplification loop
C3b
C3b
C3b
C3b
C3-activation
the amplification loop
C3b
C3b
C3b
Control of spontaneous
C3 activation via DAF
DAF prevents
C3b
the binding of
factor B to C3b
CR1
Autologous cell membrane
Control of spontaneous
C3 activation via CR1
C3b
CR1
Autologous cell membrane
C3b
iC3b
CR1
C3b stabilization and
C5 activation
C3b finds an activator
(protector) membrane
This is stable C5 convertase
of the alternative pathway
C3b
b
C3 b
C3b regulation on self and
activator surfaces
C3b
C5-convertase of the two
pathways
C5-convertase of the
Classical and lectin
Pathways
C4b
C3b
C5-convertase of the
Alternative Pathway
C3b
C3b
Lytic pathway
Generation of C5 convertase
leads to the activation of the
Lytic pathway
Components of the lytic pathway
C7
C6
C
9
Lytic pathway
C5-activation
b
C4b
C3b
Lytic pathway
assembly of the lytic complex
C6
C7
b
Lytic pathway:
insertion of lytic complex into cell membrane
C6
C7
CC C C
C9 9 9 9C
9C C C9
9 9 9
b
Biological effects of C5a
Opsonization and phagocytosis
Biological properties of C-activation
products
Product
C2b
(prokinin)
C3a
(anaphylatoxin)
Biological Effects
edema
mast cell degranulation;
enhanced vascular
permeability;
anaphylaxis
Regulation
C1-INH
carboxypeptidase- B
(C3-INA)
Biological properties of C-activation
products
Product
Biological Effects
Regulation
C3b
(opsonin)
opsonization;
phagocyte activation
factors H & I
C4a
(anaphylatoxin)
as C3, but less
potent
(C3-INA)
C4b
(opsonin)
opsonization;
phagocytosis
C4-BP,
factor I
Biological properties of C-activation
products
Product
Biological Effects
Regulation
C5a
(chemotactic
factor)
anaphylactic as C3, but
much more potent;
attracts & activates PMN
causes neutrophil
aggregation, stimulation
of oxidative metabolism
and leukotriene release
carboxypeptidase-C
(C3-INA)
C5b67
chemotaxis, attaches
to other membranes
protein-S
Antigens
Factors Influencing
Immunogenicity
Contribution of the Immunogen
• Foreignness
• Size
• Chemical Composition
–
–
–
–
Primary Structure
Secondary Structure
Tertiary Structure
Quarternary Structure
Sequence determinants
Conformational
determinants
Factors Influencing
Immunogenicity
Contribution of the Immunogen
• Foreigness
• Size
• Chemical Composition
• Physical Form
• Degradability
– Ag processing by Ag Presenting Cells (APC)
Factors Influencing
Immunogenicity
Contribution of the Biological System
• Genetics
– Species
– Individual
• Responders vs Non-responders
• Age
Factors Influencing
Immunogenicity
Method of Administration
• Dose
• Route
– Subcutaneous > Intravenous > Intragastric
• Adjuvant
– Substances that enhance an immune response
to an Ag
Chemical Nature of
Immunogens
• Proteins
• Polysaccharides
• Nucleic Acids
• Lipids
– Some glycolipids and phosopholipids can be
immunogenic for T cells and illicit a cell
mediated immune response
Types of Antigens
T-independent
• Polysaccharides
• Properties
– Polymeric structure
– Polyclonal B cell
activation
– Resistance to
degradation
• Examples
– Pneumococcal polysaccharide, lipopolysaccharide
– Flagella
Types of Antigens
T-dependent
• Proteins
• Structure
• Examples
– Microbial
proteins
– Non-self or
Altered-self
proteins
Antigenic Determinants
Recognized by B cells and Ab
• Composition
– Proteins, polysaccharides, nucleic acids
• Size
– 4-8 residues
Antigenic Determinants
Recognized by T cells
• Composition
– Proteins (some lipids)
– Sequence determinants
• Processed
• MHC presentation (lipid presentation by MHC-like
CD1)
• Size
– 8 -15 residues
• Number
– Limited to those that can bind to MHC
Biological
Foreign bit
receptor
Consequence of
Interaction
Microbial cell wall Complement
Opsonization;
components
Complement
activation
MannoseMannose-binding
Opsonization;
containing
protein
Complement
carbohydrates
activation
Polyanions
Scavenger receptors Phagocytosis
Lipoproteins of
Gram + bacteria
Yeast cell wall
components
TLR-2 (Toll-like
receptor 2)
Macrophage
activation;
Secretion of
inflammatory
cytokines
Foreign bit
Double stranded
RNA
receptor
TLR-3
LPS
TLR-4
(lipopolysaccharid
e of Gram –
bacteria
Flagellin (bacterial TLR-5
flagella)
Biological
Consequence of
Interaction
Production of
interferon
(antiviral)
Macrophage
activation;
Secretion of
inflammatory
cytokines
Macrophage
activation;
Secretion of
inflammatory
cytokines
Immunoglobulins:
Structure and Function
Immunoglobulins:Structure and
Function
• Definition: Glycoprotein molecules that are
produced by plasma cells in response to an
immunogen and which function as antibodies
General Functions of
Immunoglobulins
• Ag binding
– Can result in protection
• Effector functions
(Usually require Ag binding)
– Fixation of complement
– Binding to various cells
Immunoglobulin Structure
• Heavy & Light
Disulfide bond
Chains
• Disulfide bonds
– Inter-chain
– Intra-chain
Carbohydrate
CL
VL
CH2
CH1
VH
Hinge Region
CH3
Immunoglobulin Structure
Disulfide bond
• Variable &
Carbohydrate
Constant
Regions
– VL & CL
– VH & CH
• Hinge Region
CL
VL
CH2
CH1
VH
Hinge Region
CH3
Immunoglobulin Fragments:
Structure/Function Relationships
Ag Binding
Complement Binding Site
Binding to Fc
Receptors
Placental Transfer
IgG
• Structure
– Monomer (7S)
IgG1, IgG2 and IgG4
IgG3
IgG
• Properties
– Major serum Ig
– Major Ig in extravascular spaces
– Placental transfer – Does not require Ag
binding
– Fixes complement Binds to Fc receptors
Phagocytes - opsonization
• K cells - ADCC
IgM
• Structure
J Chain
– Pentamer (19S)
– Extra domain (CH4)
– J chain
C 4
IgM
• Properties
– 3rd highest serum Ig
– First Ig made by fetus
and B cells
– Fixes complement
Fixation of C1 by IgG and IgM Abs
No activation
Activation
IgM
• Properties
– 3rd highest serum Ig
– First Ig made by fetus
and B cells
– Fixes complement
– Agglutinating Ig
– Binds to Fc receptors
– B cell surface Ig
Tail
Piece
B Cell Antigen Receptor (BcR)
Ig-
Ig-
Ig-
Ig-
IgA
• Structure
– Serum - monomer
– Secretions (sIgA)
• Dimer (11S)
• J chain
• Secretory
component
Secretory Piece
J Chain
IgA
• Properties
– 2nd highest serum Ig
– Major secretory Ig (Mucosal or Local Immunity)
• Tears, saliva, gastric and pulmonary secretions
– Does not fix complement (unless aggregated)
– Binds to Fc receptors on some cells
IgD
• Structure
– Monomer
– Tail piece
Tail Piece
IgD
• Properties
– 4th highest serum Ig
– B cell surface Ig
– Does not bind complement
IgE
• Structure
– Monomer
– Extra domain (CH4)
C 4
IgE
• Properties
– Least common serum Ig
• Binds to basophils and mast cells (Does not require
Ag binding)
– Allergic reactions
– Parasitic infections (Helminths)
• Binds to Fc receptor on eosinophils
– Does not fix complement
Nature of Ag/Ab Reactions
• Lock and Key Concept
http://www.med.sc.edu:85/chime2/lyso-abfr.htm
• Non-covalent Bonds
– Hydrogen bonds
– Electrostatic bonds
– Van der Waal forces
– Hydrophobic bonds
• Multiple Bonds
• Reversible
Source: Li, Y., Li, H., Smith-Gill, S. J.,
Mariuzza, R. A., Biochemistry 39, 6296, 2000
Ab formation
Hallmarks
of the
Immune Response
• Self/Non-self Discrimination
• Memory
• Specificity
Kinetics of the Ab Response
T-dependent Ag; 1o Response
LAG
LOG
PLATEAU
DECLINE
Ab Titer
• Lag phase
• Log phase
• Plateau phase
• Decline phase
Ag
Days After Immunization
Kinetics of the Ab Response
• Lag phase
• Log phase
• Plateau phase
• Decline phase
Ab Titer
T-dependent Ag; 2o Response
1o Ag
2o Ag
* Specificity
Days After Immunization
Qualitative Ab Changes during
o
o
1 and 2 Responses
• Class variation
Total
Ab
IgG Ab
IgM
Ab
– 2o - IgG, IgA or IgE
Ab Titer
– 1o - IgM
1o Ag
2o Ag
Days After Im munization
Cellular Events in 1o Response to
T-dependent Ags
• Lag
– Clonal selection
• Log
– IgM
– Class switching
• Stationary
• Decline
• Memory Cell Pool
1o Ag
IgM
IgG
Memory Cells
Cellular Events in 2o Response to
T-dependent Ags
• Lag phase
– Virgin cells
– Memory cells
Virgin B cell
• Log phase
– Pool size
– IgG, IgA or IgE
• Stationary
• Decline
– Sustained
production
IgM
IgG
Memory
Pool
Memory
Cells
IgG
Memory
Cells
Cell-Cell Interactions,
T-Independent Antigens,
CD5 B Cells, Cytokines
T Cell-B Cell Interactions
(hapten-carrier effect)
• Th cells recognize carrier, B cells
recognize hapten
• Th and B cells cooperate by
interacting
• Interactions are class II self-MHC
restricted
Uniqueness of B Cells
• Express both immunoglobulin (Ig) and
class II MHC on cell surface
• Capable of producing antibody of
same specificity as that of its surface
Ig
AND
• Capable of functioning as an antigen
presenting cell
Mechanism of HaptenCarrier
• Hapten recognized by Ig receptor
on B cell
• Hapten-carrier endocytosed
• Carrier processed and presented on
class II MHC to Th cell
• Activated Th cell produces cytokines
• Cytokines enable B cell to be
activated to produce anti-hapten
antibodies
Antigen
CD40
B
MHC II
cell
Cytokine Immunoglobulin
receptor
receptor
B
B
cell
cell
cell
B7 CD28
2. B7 expressed
4. Cytokine
binds to
cytokine
receptor,
CD40 ligand
binds to CD40
B
5. B cell activated
6. B cells proliferate, differentiate, secrete Ig
TCR
cell
cell
1. Antigen presentation to
Th cell
B
T helper
B
cell
3. Th cell is
activated
and expresses
CD40 ligand,
Cytokines
secreted
CD40
ligand
T helper
cell
Cytokine
Th cell
Class II MHC
APC
B
Th cell
cell
B cell takes up and
presents antigen
Th cells are primed by antigenpresenting cell
Th cell
Th cell
B
cell
B-T cell cooperation
B cells receive signals from T cells
B
B
cell
cell
B cells divide
B
B
B
B
cell
cell
cell
cell
Antibody forming
cell
Antibody forming
cell
Antibody forming
cell
B
memory
cell
B Cells In Secondary Responses
• Memory cells created during primary
response
• Have high-affinity Ig receptors
• Can therefore take up antigens at
much lower concentrations than other
antigen presenting cells that lack Ig
antigen receptors
Cytokines
Non-antibody proteins acting as
mediators between cells, termed:
• Monokines – mononuclear phagocytes
• Lymphokines – activated T cells,
especially helper T cells
• Interleukins – abbreviated IL with a
number
Properties of Cytokines
1. Produced by cells involved in both
natural and specific immunity
2. Mediate and regulate immune
responses
3. Secretion brief and limited
- not stored pre-formed
- synthesis initiated by gene
transcription - mRNA short-lived
- cytokines produced as needed
Properties of Cytokines
(continued)
4. Can be produced by many cell types
and act on many cell types
(pleiotropic)
5. Can have similar actions
(redundant)
Properties of Cytokines
(continued)
6. Can influence synthesis of other
cytokines
- produce cascades
- enhance or suppress production of other
cytokines
- exert positive or negative regulatory
mechanisms for immune responses
7. Influence action of other cytokines - can
be antagonistic, additive, synergistic
Properties of Cytokines
(continued)
8. Bind to receptors with high affinity
9. Cells responding to cytokine can be:
- same cell (autocrine)
- nearby cell (paracrine)
- distant cell by circulation (endocrine)
10.Cellular responses to cytokines are slow,
require new mRNA and protein synthesis
Mediators and Regulators of
Natural Immunity
• Tumor Necrosis Factor-alpha (TNF-α)
• Interleukin-1 (IL-1)
• Chemokines (Chemotactic cytokines)
• Type I Interferons (IFN-α and IFN-β)
• Interleukin-12 (IL-12)
• Interleukin-10 (IL-10)
Tumor Necrosis Factor (TNFα)
• Produced by activated macrophages
• Most important mediator of acute
inflammation in response to microbes,
especially Gram-negative bacteria (LPS)
• Mediates recruitment of neutrophils and
macrophages to site of inflammation
• Acts on hypothalamus to produce fever
• Promotes production of acute phase
proteins
Interleukin-1 (IL-1)
• Produced by activated macrophages
• Effects similar to those of TNF-α
Chemokines
• Produced by many different leukocytes
and tissue cells
• Large family of >50 substances
• Recruit leukocytes to sites of infection
• Play a role in lymphocyte trafficking
Type I Interferons (IFN-α and
β)
• IFN-α a family of many proteins
produced by macrophages, IFN-β a
single protein produced by many cells
• Both IFNs inhibit viral replication
• Both increase expression of class I
MHC
• Both activate NK cells
Interleukin-12 (IL-12)
• Produced by activated macrophages
and dendritic cells
• Stimulates production of IFN-γ
• Induces differentiation of Th cells to
become Th1 cells
• Enhances cytolytic functions of
cytotoxic T cells and NK cells
Interleukin-10 (IL-10)
• Produced by activated
macrophages, Th2 cells
• An inhibitory cytokine
• Inhibits cytokine production by
activated macrophages
• Inhibits expression of class II MHC
and costimulatory molecules on
macrophages
Mediators and Regulators of
Specific Immunity
• Interleukin-2 (IL-2)
• Interleukin-4 (IL-4)
• Interleukin-5 (IL-5)
• Interleukin-10 (IL-10)
• Interferon-gamma (IFN-γ)
Interleukin-2 (IL-2)
• Produced by Th>>Tc
• Main growth factor for T cells
T cell
B cell
Monocyte
Activation
IL-2
secretion
NK
Stimulation
of division
T cell
Stimulation
of division and IFN gamma
release (and other
Increase in NK
mediators)
Cell activity
Interleukin-4 (IL-4)
• Produced by Th2 cells
• Stimulates Ig class switching to IgE
isotype
• Stimulates development of Th2 cells
from naïve Th cells
• Promotes growth of differentiated
Th2 cells
Interleukin-5 (IL-5)
• Produced by Th2 cells
• Promotes growth and differentiation of
eosinophils
• Activates mature eosinophils
• IL-4 and IL-5 can work together
Helminths opsonized with IgE can be
killed by activated eosinophils
Interleukin-10 (IL-10)
• Produced by activated macrophages,
Th2 cells
• Inhibits production of IFN-γ by Th1
cells needed to activate macrophages
Interferon-gamma (IFN-γ)
• Produced by Th cells >> Tc and NK cells
• Numerous functions in both natural and
specific immunity
Th1 cell >
Tc cell
NK
Granulocyte
Endothelial cell
Activation
Activation
IFN gamma
secretion
Macrophage
NK
Activation
Many cell types
Weak anti-viral activity,
Stops cell division,
Stops hematopoiesis
T cell
T cell activation
B
cell
Increase in
NK
cell activity
Many cell types
Differentiation, Induction of class I
Stops cell division and class II MHC
Major Histocompatibility
Complex and T Cell Receptor
Differential expression
of MHC antigens
 Class-I expressed on all nucleated cells
in man, and also on erythrocytes in
mice.
 Class-II expressed primarily on
antigen presenting cells (dendritic
cells, macrophages and B cells, etc.)
Structure of Class I MHC
α1
NH2
NH2
Alloantigenic
sites
α2
β2
NH2
COOH
CHO
α3
Disulfide bridge
Papain cleavage
Plasma membrane
OH
P
Cytoplasm
COOH
Structure of Class II MHC
NH2
NH2
CHO
α1
β1
CHO
α2
β2
CHO
Plasma membrane
Cytoplasm
COOH
COOH
Aspects of MHC
1. MHC molecules are membrane-
bound. Recognition by T cells
requires cell-cell contact.
2. Peptide from cytosol associates with
class I MHC and is recognized by Tc
cells. Peptide from vesicles
associates with class II MHC and is
recognized by Th cells.
Aspects of MHC
(continued)
3. Although there is a high degree of
polymorphism for a species, an
individual has maximum of six
different class I MHC products and
only slightly more class II MHC
products.
A peptide must associate with a
given MHC of that individual,
otherwise no immune response can
occur. That is one level of control.
Aspects of MHC
(continued)
4. Mature T cells must have a T cell
receptor that recognizes the
peptide associated with MHC. This
is the second level of control.
5. Each MHC molecule has only one
binding site. The different
peptides a given MHC molecule
can bind all bind to the same site,
but only one at a time.
Structure of T Cell
Receptor
Alpha
chain
Beta
chain
CHO
CHO
CHO
CHO
Variable region “V”
Constant region “C”
Hinge “H”
Disulfide bridge
+
+
+
Transmembrane region
Cytoplasmic tail
Structure of T Cell Receptor
(continued)
• Hypervariable regions in V contribute
to diversity of TCR
• TCR recognizes portions of MHC
molecule and peptide bound in the
groove
• Small population of T cells has a TCR
comprised of γ and δ chains – γδ TCR
specificity differs from αβ TCR
Antigen Processing and
Presentation
What Does The B Cell
Immunoglobulin (Ig)
Receptor Recognize?
1. Proteins (conformational
determinants, denatured or
proteolyzed determinants)
2. Nucleic acids
3. Polysaccharides
4. Some lipids
5. Small chemicals (haptens)
What Does the αβ T Cell
Receptor (TCR) Recognize?
1. Only fragments of proteins (peptides)
associated with MHC molecules on
surface of cells
• Helper T cells (Th) recognize peptide
associated with MHC class II
molecules
• Cytotoxic T cells (Tc) recognize
peptide associated with MHC class I
molecules
Antigen Processing and
Presentation
• Fragmentation of protein into peptides
• Association of peptide with an MHC
molecule
• Transport to cell surface for
expression
• Different cellular pathways for
association of peptide with MHC class
I and class II molecules
Class I MHC Pathway
Peptide is presented
by MHC-I to CD8
cytotoxic T cell
Peptide passes
with MHC from Golgi
body to surface
Plasma membrane
Viral protein is made
on cytoplasmic
ribosomes
Globular viral
protein - intact
Peptide associates
with MHC-I complex
rER
Proteasome
degrades
protein to
peptides
Peptide transporter
protein
moves
peptide into ER
Golgi body
Peptide with MHC
goes to Golgi body
MHC class I alpha
and beta proteins
are made on the rER
Class II MHC Pathway
Peptide MHC-II
complex is presented
to CD4 helper T cell
Endosome fuses with
plasma membrane
Immunodominant
peptide binds
to class II MHC
Golgi
body
Globular
protein
CD4 helper T cell
Endosome
Fusion of endosome
and exocytic vesicle
Endocytosis
Lysosome
Exocytic vesicle fuses
with endosome
releasing Ii from αβ dimer
Protein is processed to
peptides in endosome
or lysosome
Class II MHC
α
Synthesis
3 chains: α,β and Ii
β Ii
Endoplasmic reticulum
Points Concerning Antigen
Processing and Presentation
1. Location of pathogen
• viruses in cytosol, MHC class I
pathway, Tc response
• extracellular bacteria, MHC class II
pathway, Th2 response, Ab formation
• intracellular bacteria, MHC class II
pathway, Th1 response
Points Concerning Antigen
Processing and Presentation
2. Peptides derived from both self and
non-self proteins can associate with
MHC class I and class II molecules.
3. Chemical nature of MHC groove
determines which peptides it will
bind.
Self MHC Restriction
• T cells recognize foreign antigen
associated with self MHC
• No value for individual to have T cells
that recognize foreign antigen
associated with foreign MHC
• Self MHC restriction occurs in thymus
Process of Self MHC
Restriction in Thymus
• T cells with TCR recognizing self MHC
molecules are retained – “positive
selection”
• Retained T cells with TCR recognizing self
peptide associated with self MHC are
eliminated – “negative selection”
• Self MHC-restricted T cells are released
Functions of Th Cells,
Th1 and Th2 Cells,
Macrophages and Tc Cells.
Immunoregulation
Critical Role of Th Cells in
Specific Immunity
• Select effector mechanisms
• Induce proliferation in appropriate effectors
• Enhance functional activities of effectors
Ag
Ag
APC
Antigen-presenting cell
Ag
Th
B cell
cell
Cytokines
Tc
cell
Granulocyte
Macrophage
Cytokines
NK
NK cell
Functions of Th1 and Th2 Cells
Th1
Th2
cell
cell
Inhibits production
IL-10
IFNγ
IL-4 IL-5
Activates
Activates
Macrophage
Mast cell
B cell
Eosinophil
Antibodies (including IgE)
Central Role of Macrophages in
Natural and Specific Immunity
• Involved in initial defense and antigen
presentation
Invading agent
Macrophage
and
have effector functions
Antigen presentation
Macrophage
Activated macrophage
Macrophage
Th
cell
Cytokines
Lymphokines
Cytokines
Anti-microbial functions
Anti-tumor functions
Detailed Functions of Macrophages
Inflammation – Fever, Production
of: IL-6, TNF-alpha, IL-1 – act as
pyrogen
Immunity
Selection of lymphocytes to be
activated:
IL-12 results in Th1 activation
IL-4 results in Th2 activation
Activation of lymphocytes:
Production of IL-1
Processing and presentation of
antigen
Reorganization of tissues,
Secretion of a variety of factors:
Degradative enzymes (elastase,
hyaluronidase, collagenase)
Fibroblast stimulation factors
Stimulation of angiogenesis
Damage to tissues
Hydrolases, Hydrogen peroxide production
Complement C3a
TNF alpha production
Antimicrobial action
O2–dependent production of:
hydrogen peroxide, superoxide,
hydroxyl radical, hypochlorous acid
O2-independent production of:
acid hydrolases, cationic proteins,
lysozyme
Anti-tumor activity produced by:
Toxic factors
Hydrogen peroxide
Complement C3a
Proteases, Arginase
Nitric oxide
TNF alpha
Cytolytic T (Tc) Cells
• Tc exiting the thymus are pre-Tc cells,
i.e. have TCR that can recognize
antigen, but are not mature and cannot
kill until “armed”
• To become armed requires two signals:
1. Recognition by TCR of specific antigen
associated with class I MHC, and
2. Exposure to cytokines (IL-2 and IFN-γ)
Mechanism of Arming Tc Cells
Class I
MHC
1. Cell expressing class I MHC
presents antigen ( )
to a pre-Tc cell
3. Th cell
makes cytokines
Pre-Tc cell
IFN
IL-2
2. Antigen-presenting
cell presents antigen in
association with
class II MHC to Th cell
T helper cell
Class II MHC
APC
4. Pre-Tc cell
differentiates to
functional Tc cell
Tc cell
5. Tc recognizes antigen on
class I MHC-expressing target cell
6. Target cell
is killed
Features of Tc Killing
• Antigen-specific
• Requires cell-cell contact
• Each Tc capable of killing many target
cells
Steps in Tc Killing
Tc cell
Tc cell
Tc cell
Target cell
Target cell
Target cell
Target cell
1. Tc recognizes antigen on
target cell
2. A lethal hit is delivered by
the Tc using agents such as
perforin or granzyme B
3. The Tc detaches
from the target cell
4. Target cell dies
by apoptosis
Regulation of Immune
Responses
• Magnitude determined by balance between
the extent of lymphocyte activation and
tolerance induced by an antigen
• Nature determined by specificities and
functional classes of lymphocytes activated
• Regulatory mechanisms may act at the
recognition, activation, or effector phases
of an immune response