DEFINITION AND PROPERTIES OF ANTIGEN
• Antigen (Ag) - any substance, which is recognized by
the mature immune system of a given organism
– antigenicity - specific reactivity with cells or
molecules of the immune system
– immunogenicity - capability to elicit an immune
response
– tolerogenicity - capability to induce
immunological tolerance
COMPLEX ANTIGENS CONSIST OF THE CARRIER AND MULTIPLE
ANTIGENIC DETERMINANTS (EPITOPES)
Antigenic determinant (epitope)
part of the antigen which are recognized
by a defined immunoglobulin (B cell receptor
or antibody) or by T cell receptor
Carrier
part of the antigen directly not
involved in connection with
antibody
These terms can only be used to describe the
interaction of particular antigenic determinant
and single immunoglobulin or T cell receptor
B cell epitope
T cell epitope
recognized by B cells
recognized by T cells
proteins
polysaccharides
lipids
DNA
steroids
etc. (many artificial
molecules)
proteins mainly (8-23
amino acids)
requires processing by APC
cell or matrix associated or
soluble
B cell activation without the help of T cells
T-INDEPENDENT ANTIGEN
TI-1
T-INDEPENDENT ANTIGEN
TI-2
B cell
Complete absence of T-cell help. No
interactions no cytokines. At high
concentration B-cell mitogens.
Simultaneous activation of BCR and other
receptors on B cells (i.e. LPS binding protein
/CD14/TLR4) induces the B cells to
proliferate and differentiate
Strong crosslinking of BCR by
repetitive polysaccharide or protein
epitopes. Cytokine help provided by T,
NK cells etc.TI-2 usually do not act as
polyclonal B cell activators.
B CELL ACTIVATION
HAPTENS
Small chemical structures cannot induce B cell response
on their own (e.g. drugs, reactive compounds)
hapten
(i.e. DNP:dinitrophenyl)
1.
+
carrier + hapten
2.
hapten
primed
+
Antibody response generated against a hapten-carrier
conjugate
carrier + hapten
carrier specific
antibodies
hapten specific
carrier + hapten
specific
Factors influencing immunogenicity I.
• Foreignness
• Size
• Genetics
– Species
– Individual
• Responders vs non-responders
• Age
Factors influencing immunogenicity II.
• Dose
• Route
– Subcutaneous > intravenous,> oral > intranasal
• Adjuvant
– substances that enhance an immune response to an
antigen
(alum, LPS, Freund’s adjuvant, TLR ligands)
COMPLEX EFFECTS
depo effect – slow antigen intake by antigen presenting cells
activation of innate immunity
Factors influencing immunogenicity II.
• Physical status
- corpuscle (cell, colloid) or soluble
- denaturated or native
• Degradability
- antigen presentation by APC
Superantigens
Microbial proteins that bind to and activate all the T cells in an individual that
express a particular set or family of TCR molecules
conventional antigen
monoclonal/oligoclonal
polyclonal
T cell response
1:104
-
T cell response
1:105
107 – 108 / 1012
superantigen
activated T cells
1:4 - 1:10
1011 / 1012
Immunoglobulins
Amount of protein
• Definition:
Glycoprotein molecules that are present on
B cells (BCR) or produced by plasma cells
(antibodies) in response to an immunogen
+
albumin
globulins
α1
α2
β
Mobility
γ
Immune serum
Antigen adsorbed serum
Immunoglobulin Structure
• heavy and light
chains
• disulfide bonds
– inter-chain
– intra-chain
disulfide
bond
carbohydrate
CL
VL
CH1
VH
CH2
hinge region
CH3
Immunoglobulin Fragments:
Structure/Function Relationships
antigen
binding
complement binding site
binding to Fc receptors
placental transfer
Immunoglobulin Structure
• variable and
constant regions
• hinge region
disulfide
bond
carbohydrate
• domains
– VL & CL
– VH & CH1 - CH3
(or CH4)
• oligosaccharides
CL
CH1
VH
CH2
hinge region
CH3
Ribbon structure of IgG
Immunoglobulin Fragments
Structure/Function Relationships
• Fab
papain
– antigen binding
– valence = 1
– specificty determined
by VH and VL
• Fc
Fc
– effector functions
Fab
Immunoglobulin Fragments:
Structure/Function Relationships
• Fab
pepsin
– antigen binding
• Fc
– effector functions
• F(ab’)2
- Bivalent!
Fc
peptides
F(ab’)2
Why do antibodies need an Fc region?
the (Fab)2 fragment can •
detect antigen
•
precipitate antigen
• block the active sites of toxins or pathogen-associated molecules
•
block interactions between host and pathogen-associated
molecules
but can not activate (role of Fc region)
•
inflammatory and effector functions associated with cells
•
inflammatory and effector functions of complement
•
the trafficking of antigens into the antigen processing pathways
Immunoglobulin Structure-Function Relationship
• cell surface antigen receptor on B cells
allows B cells to sense their antigenic environment
connects extracellular space with intracellular signalling
machinery
• secreted antibody
neutralization
opsonization
complement fixation
Variability in different regions of the Ig
determines Ig classes or specificity
isotype
allotype
idiotype
(Classes/subclasses)
Sequence variability of H/Lchain constant regions
Allelic variants
Sequence variability of H and Lchain variable regions
(individual, clone- specific)
Human Immunoglobulin Classes
encoded by different structural gene segments (isotypes)
•
•
•
•
•
IgG - gamma (γ) heavy chains
IgM - mu (μ) heavy chains
IgA - alpha (α) heavy chains
IgD - delta (δ) heavy chains
IgE - epsilon (ε) heavy chains
light chain types
• kappa (κ)
• lambda (λ)
PRODUCTION OF IMMUNOGLOBULINS
BEFORE BIRTH
AFTER BIRTH
breast milk
IgA
100%
(adult)
maternal IgG
IgM
IgG
IgA
0
3
month
6
9
1 2 3 4 5 adult
year
Ig. Concentration
level
of antibodies
secondary response against
Szekunder
’lasyecondary resp
antigen A
primary response against
antigen
A response
primer
IgG
IgA
IgE
IgM
IgM
primary
response against
antigen B
5
„A” antigA
én
Antigen
10
15
20
25
30
„A” és „B”
Antigen
A and B
antigén
days
napok
napok
Polyclonal antibody response
Ag
Polyclonal
antibody
Immunserum
Set of B-cells
Ag
Activated B-cells
Antibodyproducing
plasma-cells
Antigen-specific antibodie
Ig isotype
Serum
concentration
Characteristics, functions
Trace
amounts
 Major isotype of secondary
(memory) immune response
 Complexed with antigen activates
effector functions (Fc-receptor
binding, complement activation
 The first isotype in B-lymphocyte
membrane
 Function in serum is not known
Trace
amounts
 Major isotype in protection against
parasites
 Mediator of allergic reactions (binds
to basophils and mast cells)
3-3,5 mg/ml
 Major isotype of secretions (saliva,
tear, milk)
 Protection of mucosal surfaces
12-14 mg/ml
1-2 mg/ml
 Major isotype of primary immune
responses
 Complexed with antigen activates
complement
 Agglutinates microbes
 The monomeric form is expressed in
B-lymphocyte membrane as antigen
binding receptor
Antibodies with different isotypes differ in their
Binding affinity, effector functions and their
Transport.
Carbohydrate antigens are usually recognized
By IgM type antibodies.
Differences in transport makes all the differece:
Antibodies spec. to blood group antigens
Structures of the ABO blood group antigens
Defined by specific enzymes inherited co-dominant genes (Mendelian rules)
Donors and recipients for blood transfusion
-
+
+
+
-
-
+
+
-
+
-
+
-
-
-
-
Rhesus (Rh) blood group antigen (D)
POLYPEPTIDE TYPE ANTIGEN
extracellular space
cytoplasm membrane
intracellular space
IgG type antibody
- incomplete
no direct agglutination
but human immunglobulin-reactive
2. antibody can cause agglutination
indirect agglutination
Pathological consequences of placental transport of IgG
(hemolytic disease of the newborn)
Effects of agglutination in vivo
ABO incompatibility
intravascular haemolysis
(complement mediated haemolysis)
Rh incompatibility
haemolytic disease of the
newborn (erythroblastosis
fetalis)
(opsonisation of red blood cells, which
are then phagocytosed by macrophages
and granulocytes)
Rh profilaxis