‫بسم اهلل الرحمن الرحيم‬
Antigen
Samira Rajaei, MD, PhD.
Assistant professor
Department of Immunology
Tehran University of Medical Sciences
Antigen definition
Antigen
• Antigen
• Immunogen
• Tolerogen
Anti(body)+ generator
Which cells are the main
players of adaptive
immunity?
Antigen
Any substance that may be specifically recognized by an
antibody molecule (BCR) or T cell receptor (TCR)
Difference between Ag recognition
 Antibodies can recognize as antigens almost every
kind of biologic molecule:




Lipids
Carbohydrates
Proteins
Nucleic acids
 TCRs mainly recognize peptides
Immunogen
Antigens are capable of
activating lymphocyte
True/False
All antigens are recognized by specific lymphocytes
① true ②false
All antigens are capable of activating lymphocyte
① true ②false
Tolerogen
 Foreign antigens may be administered in ways that
preferentially induce tolerance rather than immune
responses.
 These antigens are tolerogen.
Hapten
 Small chemicals (dinitrophenol) may bind to
antibodies, and are therefore antigens, but cannot
activate B cells on their own (they are not
immunogenic)
Immunogenicity
Foreignness
Molecular size
Chemical structure complexity
Dosage, Route, individual difference,
timing of administration
Foreignness
 Foreignness is essential to immunogenicity
 Self-responsive cells are eliminated during
lymphocyte ontogeny
 More Foreignness; more immunogenicity
Molecular size
 >100000 Dalton
 <10000 Dalton
Chemical structure complexity
 Composition
 Proteins (most potent)
 Polysaccharides (second potent)
 Nucleic acids and lipids (are not immunogenic unless …)
 Chemical complexity
 Homopolymrs of amino acids?
 Which one of protein structures are more immunogen?
 More complex molecules are more immunogen
Dosage
 High doses of immunogen may cause a lack of
responsiveness (tolerance)
 Intermediate doses of immunogen are generally the
best immune response inducers
 Low doses of immunogen may not induce immune
responses
Route of immunogen administration
 Route of administration influences which cells (and
how many of them) are stimulated






s.c. (subcutaneous)
i.d. (intradermal)
i.m. (intramuscular)
i.v. (intravenous)
Oral administration
i.n. (intranasal)
How to generate
antibody against
haptens?
Hapten+Carrier
 attach them to a protein before immunization which
is called carrier.
 attaching a number of hapten molecules to a single
molecule of a polysaccharide (render it multivalent)
Adjuvant
 An adjuvant is any substance that enhances the immunogenicity
of substances mixed with it
 Many adjuvants in experimental use are microbial products, such
as killed mycobacteria and LPS, that engage TLRs
 What are the differences between Adjuvant and carrier?
 do not form stable linkages with the immunogen
 adjuvants are needed primarily for initial immunizations, whereas
carriers are required to elicit not only primary but also
subsequent responses to haptens.
Physicochemical structure of
Antigens
 Soluble : (proteins , polysaccharides) + Adjuvants
 Particulate: bacteria, virus and RBCs (Adjuvants are
not necessary)
 Colloidal (non robust immunogens)
How adjuvants enhance
immunogenicity of proteins?
 Delayed release of antigens
 Adjuvants convert soluble protein antigens into
particulate material, which is more readily ingested by
antigen-presenting cells such as macrophages
 adjuvants activate dendritic cells to express more
MHC, increase the expression of costimulators, and
cytokines needed for T cell activation, stimulate
migration of the dendritic cells to lymph nodes .
Types of adjuvants
 Freund’s adjuvant
 Incomplete (oil in water emulsion)
 Complete (oil in water emulsion and dead
mycobacteria)
 Alum (aluminum hydroxide or aluminum phosphate)
Epitope/Determinant
Macromolecules (proteins, polysaccharides,
and nucleic acids) are usually much bigger
than the antigen-binding region of an
antibody molecule
Any antibody binds to only a
portion of the
macromolecule, which is
called a determinant or
an epitope
 Most of the macromolecules have multiple
determinants.
Polyvalency/multivalency
 The presence of multiple identical determinants in an
antigen is referred to as polyvalency or multivalency.
 Polysaccharides
 nucleic acids
 Surface of microbs
 Polyvalent antigens can induce
clustering of the B cell receptors
and initiate the process of B cell
activation
Globular proteins
 Most globular proteins do not contain multiple
identical epitopes and are not polyvalent
 Proteins could not cross link the B cell receptors.
 They need the help of T lymphocytes.
 Proteins are T-dependent antigens.
Spatial arrangement of epitopes
 Nonoverlapping determinants
 2 or more antibody bind to a protein antigen
 No steric interference
 Overlapping determinants
 Steric interference
Allosteric Effects
 Allosteric effects (conformational changes positively
or negatively effects another antibody binding
without Steric interference)
Linear determinants
 Epitopes formed by several adjacent amino acid residues
are called linear determinants.
 The antigen-binding site of an antibody can usually
accommodate a linear determinant made up of about 6
amino acids.
 If linear determinants appear on the external surface or in
a region of extended conformation in the native folded
protein, they may be accessible to antibodies
 More often, linear determinants may be inaccessible in the
native conformation and appear only when the protein is
denatured.
Conformational determinants
 Conformational determinants are formed by amino
acid residues that are not in a sequence but become
spatially juxtaposed in the folded protein.
Neoantigenic determinants
 Proteins may be subjected to modifications such as
glycosylation, phosphorylation, ubiquitination,
acetylation, and proteolysis.
 Modifications produce new epitopes.
Epitopes determined by T cells
 T lymphocytes recognize linear peptide determinants
in combination with MHC residues.
Mitogens
 These molecules are able to bind to TCR complex and
CD2, causing them to cluster on the cell surface,
thereby mimicking the clustering caused by antigen
presentation.
 They activate the T cells regardless of their antigen
specificity.
 Phytohemagglutinin (PHA), a lectin
 Concanavalin-A
Superantigens
 Molecules that can activate T cells non-specifically
 Superantigens bind to MHC class II molecules on APCs
and are recognized by TCRs, but not in the same way
as an MHC molecule-peptide complex.
 Binding is to the Vβ chain of the TCR
 Staphylococcal enterotoxins
 Toxic shock syndrome toxin