Antigens
Objectives
• What is an antigen?
• What type of molecules are antigens?
• Can any particle/foreign body that enters the cell be an
antigen? Properties and features of antigens that make
them immunogens.
• To understand that antigens can be recognized by both B
and T cells.
• The structure of antibodies that facilitates this interaction.
• To be familiar with the fact that there are different classes
of antibody molecules and they have slightly different
structures that determine function.
• With so many antigens how do we generate so many
antibodies, TCR that recognize them?
ANTIGENS
1. A foreign substance or component of an
organism.
2. Antigens are substances that induce a
specific immune response and
subsequently react with the products of
a specific immune response.
3. Any substance against which an
antibody is produced.
Antigenicity
The ability to bind antibody or T cell
receptor.
Immunogenicity
The capacity to stimulate the production of
antibodies or cell-mediated immune
responses.
Antigen vs. Immunogen
• Antigen
– Any substance that can
bind to an antibody or T
cell receptor
• Immunogen
– Any substance that can
elicit an immune
response
– All immunogens are
antigens
– Not all antigens are
immunogens, i.e.
haptens
• Tolerogen - An antigen that invokes a specific
immune non-responsiveness due to its
molecular form. If its molecular form is
changed, a tolerogen can become an
immunogen.
• Allergen - An allergen is a substance that
causes the allergic reaction. The (detrimental)
reaction may result after exposure via
ingestion, inhalation, injection or contact with
skin.
Antigens can be classified in order of their origins
• Exogenous antigens
Exogenous antigens are antigens that have
entered the body from the outside, for example
by inhalation, ingestion, or injection.
By endocytosis or phagocytosis, these antigens
are taken into the antigen-presenting cells (APCs)
and processed into fragments.
Types of Antigens
Exogenous Antigens
1- Bacterial antigens:
a- Antigens related to bacterial cells
- Somatic antigen (O): part of cell wall gm –ve bacter.
- Capsular antigen: usually polysaccharide
- Flagellar Ag (H) : a protein made of flagellin
- Fimbrial Ag: surface antigens in fimbriated bacilli
b- Antigen secreted by bacteria:
- Exotoxins
- Enzymes
2- Viral antigens:
a- protein coat viral antigens
b- Soluble antigens (soluble nucleoproteins as in influenza)
• Endogenous antigens
Endogenous antigens are antigens that have
been generated within the cell, as a result of
normal cell metabolism, or because of viral or
intracellular bacterial infection.
Types Of Antigens
Endogenous antigens
Human tissue antigens:
a- Blood group antigens:
A, B and Rh antigens
b- Histocompatibility antigens:
Glycoprotein molecules on all nucleotide cells:
- Major histocompatibility complex antigens (MHC)
- Human leucocyte antigen (HLA)
Antigen Binding And Recognition Molecules
Antigens are recognized by and bind to:
1) B-cell receptors (BCR) :
- These are membrane-bound immunoglobulins
(IgM and IgD) on B-cells
- BCRs can be secreted in plasma as antibodies
2) T-cell receptors (TCR)
- α and β chains anchored to T-cells
- There is a groove which binds small peptides
presented by MHC on surface of APCs
3) MHC molecules
They are essential for presentation of peptides so that they can be
recognized and bind to TCRs
Table from Cellular and Molecular Immunology Chapter 4 page 76
CHEMICAL NATURE OF IMMUNOGENS
• Proteins
The vast majority of immunogens are proteins. These may be pure
proteins or they may be glycoproteins or lipoproteins. In general,
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.
Additional Antigen Types
• We will discuss the following when we talk
about T cells:
– Major Histocompatibility Complex (MHC)
– Super antigens
Immunogens or Antigens
Haptens:
- Low molecular weight substances
- These substances not immunogenic by itself
- If couple to a larger carrier molecule (albumin, globulins), they
become immunogenic
- Examples :
simple chemicals and drugs:
penicillin, sulphonamid, aspirin, cosmetic, tranquillizers, neomycin
skin ointment
T-independent and T-dependent
Antigens
• Just another way to group antigens.
T-independent Antigens
T-independent antigens are antigens which
can directly stimulate the B cells to produce
antibody without the requirement for T cell
help In general, polysaccharides are Tindependent antigens. The responses to these
antigens differ from the responses to other
antigens.
Properties of T-independent antigens
• Polymeric structure
These antigens are characterized by the same antigenic
determinant repeated many times
• Polyclonal activation of B cells
Many of these antigens can activate B cell clones
specific for other antigens (polyclonal activation).
– Type 1 T-independent antigens are polyclonal activators
– Type 2 are not.
• Resistance to degradation
T-independent antigens are generally more resistant to
degradation and thus they persist for longer periods of
time and continue to stimulate the immune system.
T-dependent Antigens
T-dependent antigens are those that do not
directly stimulate the production of antibody
without the help of T cells. Proteins are Tdependent antigens. Structurally these
antigens are characterized by a few copies of
many different antigenic determinants.
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 activate a large fraction of
the T cells (up to 25%). These antigens are called superantigens
• Examples of superantigens include: Staphylococcal enterotoxins (food
poisoning), Staphylococcal toxic shock toxin (toxic shock syndrome),
Staphylococcal exfoliating toxins (scalded skin syndrome) and
Streptococcal pyrogenic exotoxins (shock).
• Although the bacterial superantigens are the best studied there are
superantigens associated with viruses and other microorganisms as
well.
• 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.
Superantigens (SAgs)
* They activate multiple clones of T-lymphocytes
* Bacterial toxins:
Staph. aureus toxic shock syndrome toxin (TSST) and enterotoxins
Strpt. pyogenes pyrogenic toxin A
* They have the ability to bind both class II MHC molecules and TCR
β chain
* They act as a clamp between the two, providing a signal for T-cell
activation
Superantigens (SAgs)
* They are active at very low concentration causing release of large
amounts of cytokines
* The massive T-cell activation and release of large amounts of
cytokines cause systemic toxicity
* This method of stimulation is not specific for the pathogen
* It does not lead to acquired immunity i.e no memory
Conventional Ag
Incomplete antigens have antigenic
determinants, but cannot induce immune
responses because they lack one or more of the
important attributes needed for this function
(one example of an incomplete antigen is a
hapten, which is an artificial monovalent
epitope)
• Autoantigens
An autoantigen is usually a normal
protein or complex of proteins (and
sometimes DNA or RNA) that is
recognized by the immune system of
patients suffering from a specific
autoimmune disease.
These antigens should under normal conditions not
be the target of the immune system, but due to
mainly genetic and environmental factors the
normal immunological tolerance for such an
antigen has been lost in these patients.
Common Experimental Antigens
Major Histocompatibility Complex Antigens
(MHC)
* MHC has an important function in presentation of antigens to Tcells
* Helper T-cells recognize foreign antigens on surface of APCs, only
when these antigens are presented in the groove of MHC II
molecule
* Cytotoxic T-cells will only recognize antigens, on the surfaces of
virus infected cells or tumor cells only when these antigens are
presented in the groove of Class I molecule (MHC restriction)
Mitogen
• An agent that induces mitosis.
Here means to activate T cells and/or B
cells without help from APCs.
• E.g Lectin, for example, concanavalin A
(ConA), LPS(lipopolysaccharide),
Staphylococcal protein A(SPA)
Immunogenicity of Antigens
• Determined by
–
–
–
–
Foreignness
Molecular Size
Chemical Composition
Degradability
• Influenced by
– Host genotype
– Dose and route of
administration
– Presence of adjuvants
Foreignness
The immune system normally discriminates
between self and non-self such that only
foreign molecules are immunogenic.
Size
There is not absolute size above which a
substance will be immunogenic. However, in
general, the larger the molecule the more
immunogenic it is likely to be.
Chemical Composition
In general, the more complex the substance is
chemically the more immunogenic it will be.
The antigenic determinants are created by the
primary sequence of residues in the polymer
and/or by the secondary, tertiary or
quaternary structure of the molecule.
Physical form
In general particulate antigens are more
immunogenic than soluble ones and
denatured antigens more immunogenic than
the native form.
Degradability
Antigens that are easily phagocytosed are
generally more immunogenic. This is because
for most antigens (T-dependant antigens, see
below) 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).
Contribution of the Biological System
• Genetic Factors
– Some substances are immunogenic in one species but not
in another.
– Some substances are immunogenic in one individual but
not in others (i.e. responders and non-responders).
– The species or individuals may lack or have altered genes
that code for the receptors for antigen on B cells and T
cells or they may not have the appropriate genes needed
for the APC to present antigen to the helper T cells.
• Age
Usually the very young and the very old have a
diminished ability to mount and immune response in
response to an immunogen.
Method of Administration
• Dose
The dose of administration of an immunogen can influence
its immunogenicity. There is a dose of antigen above or
below which the immune response will not be optimal.
• Route
Generally the subcutaneous route is better than the
intravenous or intragastric routes. The route of antigen
administration can also alter the nature of the response
• Adjuvants
Substances that can enhance the immune response to an
immunogen are called adjuvants. The use of adjuvants,
however, is often hampered by undesirable side effects
such as fever and inflammation.
Summary of Factors influencing
Immunogenicty
1-Foreigness :
Foreign substances are immunogenic
2- Molecular size:
High molecular weight increase immunogenicty
3- Chemical structure complexity:
High complexity increase immunogenicty
4- Route of administration:
Parenteral routes are more immunogenic to oral route
Antigenic epitopes
Epitope,or,Antigenic determinants, are
the portions of antigen molecules that
physically interact with paratopes
(combining sites) of immune response
molecules and therefore actually
"determine" antigen specificity
Epitopes
• Discrete sites on immunogens recognized by
antibodies or T cell receptors
• Macromolecules contain many epitopes with
different immunogenicities
• Immunogenicity can vary between individuals and
populations.
• Some epitopes are immunodominant
Epitopes
• Overlapping
• Non-overlapping
• Linear
• Conformational
Antigenic epitopes
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.
• 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.
Specificity depends upon conformation, or
three-dimensional shape, which is a
combination of tertiary and quaternary
structure ... supported by primary and
secondary structure, of course.
Typical size is hard to pinpoint, but sequences of
up to 16 amino acids in certain protein
antigens have been shown to interact with
their complementary paratope.
Two different epitopes
• B cell epitope, a portion
• T cell epitope, the
of antigen molecule
region of antigen
that is recognized by B
molecules that are
cell receptors.
recognized by T cell
receptors.
Classification of Antigens
• Thymus-dependent antigen(TD-Ag)
• Thymus-independent antigen(TI-Ag)
DETERMINANTS RECOGNIZED BY THE
INNATE IMMUNE SYSTEM
• Let’s not forget we have the innate immune system too that
recognizes antigens.
• Determinants recognized are different.
• Innate immune system recognize broad molecular patterns
found in pathogens but not in the host.
• The broad molecular patterns recognized by the innate
immune system have been called PAMPS (pathogen
associated molecular patterns) and the receptors for
PAMPS are called PRRs (pattern recognition receptors).
• A particular PRR can recognize a molecular pattern that
may be present on a number of different pathogens
enabling the receptor to recognize a variety of different
pathogens.
Adjuvant
• Adjuvant: The Latin "adjuvans" means to help,
particularly to reach a goal.
• An adjuvant is a substance that helps and
enhances the pharmacological effect of a drug or
increases the ability of an antigen to stimulate
the immune system.
Classification of Adjuvant
• Freund’s adjuvant
– Complete Freund’s adjuvant(CFA)
– Incomplete Freund’s adjuvant(IFA)
•
•
•
•
Liposome
Inorganic compound
Cytokine
Biodegradable nanoparticles
Mechanisms of adjuvants
• Prolonged persistence of immunogen
molecules at the site of injection.
• Enhancement of co-stimulatory signals.
• Induction of granuloma formation.
• Stimulation of lymphocyte proliferation in a
non-specific manner.