I. Definition of antigen
Antigen is substance which when introduced
parentally into the body stimulates the
production of an antibody with which it reacts
specifically and in an observable manner
Antigen:
Immmunogen
Tolerogen
Allergen
Vaccine
1. Immunogen: the antigen that induce specific immune
response
Microbes; bacteria ,virus; fungi and parasites
xenoantigenei or allogeneic tissues or organs:
grafted skin , bone marrow
2.Tolerogen: antigen that induce Immunologic tolerance
•
Immunologic tolerance is unresponsiveness to an antigen
that is induced by prior exposure to that antigen.
tolerogen
3. Allergen: antigen that induce Anaphylaxis
(severe immediate hypersensitivity reaction
occurring as a result of rapid generalized mastcell granulation)
Allergen: some medicine, flower powder, seafood
4. Vaccine: antigens that induce a protection
immune response against microbes and are
used to prevent diseases
Killed vaccine: Rubella virus
Attenuated vaccine: Measles
Toxoid :Tetanus
Based on Immunogenicity
Complete antigen : substances with both
immunogenecity and immunoreactivity
By convention , we call complete antigen as antigen.
Incomplete antigen (hapten): substances only with
immunoreactivity
Hapten +carrier
complete antigen (immunogens)
Hapten: Only possess immunoreactivity
Carrier: Make hapten obtain the immunogenicity
2. Two properties of immunogen
1. Immunogenicity
An ability of antigen which can stimulate the body
to evoke a specific immune response.
2. Immunoreactivity
An ability of antigen which can combine with
corresponding Ab or sensitized lymphocyte
Based on Chemical nature
• Proteins
Majority of immunogens are proteins (pure proteins or they
may be glycoproteins or lipoproteins). Proteins are usually
very good immunogens.
• Polysaccharides
Pure polysaccharides and lipopolysaccharides are good
immunogens.
• Nucleic Acids
Nucleic acids are usually poorly immunogenic. However,
they may become immunogenic when single stranded or
when complexed with proteins.
• Lipids
In general lipids are non-immunogenic, although they may
be haptens.
According to source of antigens
• Xenoantigen
• Alloantigen
• Autoantigen
• Heterophile antigen
(1) Xenoantigen
• An antigen that is found in more than one species. An antigen is something
that is capable of inducing an immune response.
• The prefix "xeno-" means foreign or other. It comes from the Greek "xenos"
meaning stranger, guest, or host.
• Pathogens: bacteria, virus , fungi, parasite
• Exotoxin and toxoid
Exotoxin
Produced by G+ bacteria
Strong antigenicity and pathogenicity
Toxoid :
exotoxin that loses its toxicity but maintains its antigenicity under suitable
conditions (low concentration of formaldehyde )
Such as tetanus toxoid , diphtheria toxoid
bacteria
Pathogens
Fungi
HIV
Heterophile Ag (forssman Ag)
 Common Ags shared by different species ( between human and
animal or microbes, between different species of microbe)
(eg) M protein of streptococus bears common antigen
determinant with basement membrane of kidney
(This common between bacteria and human being can causes poststreptococcal
glomerulonephritis)
 No specificity of species
 Significance . immunopathology
. Diagnosis
(2) Alloantigen
Antigens of red blood cell
•
ABO system (blood typing)
- very important in transfusion
•
Rh system (Han race :>99%Rh+)----haemolytic disease of
the newborn (HDNB)
HLA system (Human leukocyte antigen)
- relate to transplantation
- very important in immune regulation
ABO system
Blood
typing
A
B
AB
O
antigen
of RBC
A
B
A,B
-
antibody in
serum
anti-B
anti-A
anti-A, anti-B
(3) Autoantigen
Release of sequestered Ag
Lens protein is released into blood to induce immune response
to induce inflammation of lens
Change of molecular structure of auto-tissues
Denatured IgG becomes antigen to induce production of antibody
( rheumotoid factor)
In patient suffering from rheumotoid arthritis
II. According to whether need the help of T cells
when B cells produce Ab
TD-Ag (thymus dependent Ag )
TI-Ag (thymus independent Ag)
1.TD-Ag (thymus dependent Ag )
TD-Ag can stimulate B cell to produce Ab with
the help of T cell





The most of TD-Ag belong to protein
many kinds of determinants
stimulate B cell to produce :IgG, IgM, IgA
capable of inducing CMI
immune memory
2. TI-Ag (thymus independent Ag)
TI-Ag can stimulate B cells to produce Ab without the
help of T cell
 most are polysaccharide




more ,same, repeat determinant
only induce B cell to produce IgM
can not induce CMI
no immune memory
SUPERANTIGENS
• When the immune system encounters a
conventional T-dependent antigen, only a small
fraction (1 in 104 -105) of the T cell population is
able to recognize the antigen and become activated
(monoclonal/oligoclonal response).
• However, there are some antigens which
polyclonally activate a large fraction of the T cells
(up to 25%). These antigens are called
superantigens
Eg:
Staphylococcal
enterotoxins
(food
poisoning), Staphylococcal toxic shock toxin
(toxic
shock
syndrome),
Staphylococcal
exfoliating toxins (scalded skin syndrome) and
Streptococcal pyrogenic exotoxins (shock).
The diseases associated with exposure to
superantigens are, in part, due to hyper activation
of the immune system and subsequent release of
biologically active cytokines by activated T cells.
• Tumor specific Ag ( TSA)
Only expressed on the tumor cells but normal cells
The tumor antigens encoded by genomes oncogenic virus
EB virus ---B cell lymphoma HPV-cervical carcinoma
• Tumor associated Ag (TAA)
Highly expressed on tumor cells but lowly
normal cells, such as AFP CEA
expressed on
AFP (alpha-fetoprotein): over-expression in liver cancer
CEA (carcinoembryonic antigen): over-expression in carcinoma of
colon , pancreas, stomach ,and breast
DEPEND ON SIZE
• Very high molecule haemocyanin (MW
6.75million) –highly antigenic
• Low molecular weight molecule (<10,000) non antigenic or feeble
They render antigenicity by absorbing
inert particles like bentonite or kaolin
1. Antigen must be foreignness to immune system:
.
What substances are foreignness to immune system ?
According to Burnnet’s clone selection theory ,
foreignness ( non-self) means substances that never
contact with lymphocytes during embryo period.
What kinds of substances can be foreignness to immune system?
(1) Heterogeneous substances
Various pathogens, xenoantigeneic tissues
(2) Allogeneic substance
grafted allogeneic tissues or organs
(3)Autoantigenic components that never contact with lymphocytes during
embryo period

Release of sequestered antigen-----Such as lens protein, sperm etc.

Change of molecular structure of auto-tissue
For example, denatured IgG in patient suffering from rheumatoid arthritis
becomes antigen to induce production of antibody ( rheumatoid factor)
III. Specificity and cross reaction of antigen
• Specificity is a cardinal feature of the adaptive immune
system
• Specificity is referred to that immune responses are
directed toward and able to distinguish between distinct
antigen or small parts of macromolecular antigens.
• This fine specificity is attributed to lymphocyte antigen
receptors that may bind to one molecule but not to
another with only minor structural differences from the
first
Specificity of Ag
Ab1
Ab2
Ab3
• Specificity exists in both immunogenecity
and immunoreactivity
• Specificity is the basis of immunologic
diagnosis and immunologic therapy as well
as basic feature of adaptive immunity
Degradability
• Antigens that are easily phagocytosed are
generally more immunogenic. This is
because for most antigens (T-dependant
antigens) the development of an immune
response requires that the antigen be
phagocytosed, processed and presented to
helper T cells by an antigen presenting cell
(APC).
1. Antigen determinants (epitope)
(1) The portion of antigen molecules which can be
specifically recognized by antibody or antigenic receptor
of lymphocytes.
• Protein antigen----5-15 amino acid residues
• Polysaccharide antigen----5-7 polysaccharide residues
Properties of Epitopes
• They occur on the surface of the protein
and are more flexible than the rest of the
protein.
• They have high degree of exposure to the
solvent.
• The amino acids making the epitope are
usually charged and hydrophilic.
Methods to identify epitopes
1.
Immunochemical methods
•
•
•
2.
3.
ELISA : Enzyme linked immunosorbent assay
Immunoflurorescence
Radioimmunoassay
X-ray crystallography: Ag-Ab complex is crystallized
and the structure is scanned for contact residues between
Ag and Ab. The contact residues on the Ag are
considered as the epitope.
Prediction methods: Based on the X-ray crystal data
available for Ag-Ab complexes, the propensity of an
amino acid to lie in an epitope is calculated.
• In protein antigens epitopes can be defined in terms of:
– Amino acid composition
– Protein location
– Length (5-15 amino acids)
• Immunodominant epitopes:
– Epitopes bound by a greater proportion
of antibodies than others in a normal
in vivo immune response.
– Also known as Major Antigenic Sites.
• Epitopes can be divided into 2 classes:
– Discontinuous epitopes
– Continuous (linear) epitopes
(2) A change of antigenic determinant (characteristics,
number and conformation) can influence the
specificity of Ag.

Antigen determinant is the sites of Ag combining with
Ab
The chemical component , arrangement and conformation
affect the specificity of antigen
Antigen-antibody complexes are held together by noncovalent forces (therefore, antigen binding by antibody is
reversible)
2. Antigenic valence
Total number of determinants which can be bound by
antibody or antigenic receptor of lymphocytes is
called antigenic valence.
Most natural antigens are polyvalence antigen.
3. Classification of antigenic determinant
(1)According to the structure of Ag determinants
• Conformational determinants
• Sequential (or linear) determinants
Conformational determinants
Conformational determinants
are formed by amino acid
residues that aren’t in a
sequence but become spatially
juxtaposed in the folded protein.
Sequential (or linear) determinants
Epitopes formed
by several adjacent
amino acid residues
are called linear
determinants.
(2)According to types of cells recognizing antigenic
determinants
• T cell determinants (T cell epitopes)
• B cell determinants (B cell epitopes)
Difference between T cell epitope and B cell epitope
T cell epitope
Receptor
TCR
Nature
short peptide
Size 8-17 amino acid residues
B cell epitope
BCR
proteins, polysaccharides
5-15 amino acid residues
or 5-7 monosaccharides
Types
linear epitope
conformational epitope
or linear epitope
Position any position in antigen
mostly exist on the surface of
antigen
Antibodies, unlike TCRs, bind continuous (linear) or
conformational epitopes
• Usually antigens recognized by antibodies are in
their native configuration. Antibodies bind on the
surface of the antigen (e.g., amino acids that are
buried in the center of a globular protein are not
accessible by antibodies).. Antibodies bind linear
and conformational epitopes.
• Antibodies can bind proteins, carbohydrates, lipids and
nucleic acids (essentially any macromolecule). (most
antibodies bind proteins or carbohydrates)
Unlike antibodies, the TCR binds only continuous
(linear) epitopes
• The TCR binds peptides from processed protein (proteins that have been
chopped-up). Accordingly, TCRs can bind peptides that are derived
from the surface of a proteins or peptides from the interior of the protein.
• Because of processing and presentation of peptides, the
TCR binds only linear epitopes. TCRs can bind non-surface
epitopes buried in the center of a big protein and can bind
peptides from proteins from inside of bacteria and viruses.
TCRs bind only peptides [not nucleic acids, not lipids and
not carbohydrates (rare exceptions)] because MHC presents
only peptides (not nucleic acids, not lipids, not
carbohydrates).
Immunological Synapse
APC
T cell
ICAM-1,2
LFA-1
adhesion
LFA-3
CD2
CD4 or CD8
TCR signaling
antigen/MHC
TCR
CD28
B7-1,2
Co-stimulation
CD40L
LFA-1
TCR
Immature immunological
synapse
CD40
TCR
LFA-1
Mature immunological
synapse
TCR down-regulation
Co-stimulation of T cell activation
Induced by inflammation
(TLR activation, TNF-α, IL-1)
B7-1 (CD80)
CD28
Constitutive expression
B7-2 (CD86)
APC
T cell
WT(+LPS)
WT(-LPS)
CD28-/-
Defective IL-2 production in the absence
of CD28 (+ or - LPS)
CD28 is required for T cell activation
CD28 stimulates a subset of TCR
signaling pathways.
CD40L-CD40 interaction stimulates B7 expression.
Antigen/MHC
CD40 is expressed on APCs
(DC, B and Mφ)
B7
TCR
Activation of T cell
T
CD28
CD40
Antigen/MHC
B7
CD40
Antigen/MHC
Further induction of B7
Stronger co-stimulation
B7
CD40
TCR
Induction of CD40L
T
CD28
CD40L interacts with CD40.
CD40L
TCR
CD28
CD40L
T
CD40 or CD40L deficiency
reduces T cell activation.
CTLA-4 down-regulates co-stimulation.
CD28
B7
T cell
CD28 is expressed on
resting T cells.
activation
APC
B7
CD28
T cell
CTLA-4 is induced after
T cell activation.
CTLA-4 is homologous to CD28.
APC
CTLA-4 deficiency leads to
Lymphoproliferation disorder
The mice die 3-4 weeks after birth.
The mice suffer massive tissue destruction.
activation
Hyper-proliferation of T cells
CTLA-4
B7
APC
T cell
CTLA-4 binds B7 tighter than
CD28.
CTLA-4 may also send a negative
inhibition signal through intracellular domain.
3.Common antigen and cross reaction
(1) Common antigen ( common determinants in fact )
Different bacteria which possess the same epitopes are
called common antigen.
(2) Cross reaction
---Existence of common determinants
Because there are some common antigen determinants existing in
different microbes, so the antiserum against one kind of microbe
can also react with another microbe,this called cross reaction.
flagellum
Typhoid
bacillus
Paratyphoid
bacillus
2
1
H Ag
2
A
3
B
O Ag
Anti-typhoid
serum
Anti-Ag2
Anti-Ag2
Anti-Ag1
AntiParatyphoid
serum
Anti-Ag3
(3) Significance
In clinic, existence of cross reaction may lead to
wrong diagnosis.
Flu virus
typhoid
What you should know by the end of this
lecture
 Definition and characteristics of antigen
 Definition of antigenic determinants,conformational
determinants and linear determinants
 Difference between T cell epitopes and B cell epitopes
 Definition of common antigen and cross reaction
 Difference between TD-Ag and TI-Ag
 Immunological synapse.