BIOT 307
Kuby, Ch. 3, Antigens
March, 2013
General Introduction
• Specificity due to recognition of antigenic
determinants or epitopes
• Epitopes = immunologically active regions that
bind to:
– Ag-specific membrane receptors on lymphocytes
– Secreted antibodies
• Multiple epitopes can be found on single
macromolecule
ANTIGENS = SUBSTANCES
RECOGNIZED BY
• immunoglobulin receptor of B cells
• T- cell receptor when complexed with MHC
• Host determines whether B or T cell’s antigenbinding receptor actually induces immune
response
• B and T cell recognition are different
Immunogenicity and Antigenicity
• Immunogenicity = ability to induce humoral
and/or cell- mediated immune response
• Antigenicity = ability to combine specifically
with the final products of the above responses
(i.e., antibodies and/or cell-surface receptors).
• Haptens, small molecules, are antigenic but
incapable of inducing specific immune
response
FACTORS INFLUENCING
IMMUNOGENICITY
• Type of molecule:
– Protein and polysaccharides immunogenic
– Lipids and nucleic acids must be conjugated with
another molecule, i.e., like above
PROPERTIES DETERMINING
IMMUNOGENICITY
Foreignness or non-self
• Non-self antigens are eliminated by recognition
and response
• lymphocytes that recognize self antigens are
eliminated by inactivation
• Antigens that are more foreign are more
immunogenic
– Exceptions:
• Collagen and cytochrome c – similar in all species
• Corneal tissue and sperm - kept apart from immune system
Molecular Size
• > 100,000 Da: excellent
• < 5,000-10,000: poor
Chemical Composition and Heterogeneity
• Homopolymers: poor
• Protein structure: 1’, 2’, 3’, 4’ all contribute
• Lipids serve as haptens  Ab against lipids
useful – leukotrienes, steroids, vitamins
Susceptibility to Ag processing and presentation
• Interaction of T-cells with processed Ag
• Processing by: MΦs, neutrophils
• Presentation via APCs (MΦs, DCs, B cells,
mast cells)
• Larger molecules more easily phagocytosed
HOST CONTRIBUTES TO
IMMUNOGENICITY
• Genes in MHC
• Genes that encode B and T-cell receptors
• Genes involved in immunoregulation
EVIDENCE - Mice
• Inbred strains of mice respond differently to Ag
• F1 generation – intermediate
• Backcross analysis mapped differences to MHC
subregion
HOST CONTRIBUTES TO
IMMUNOGENICITY
EVIDENCE – Humans
• HLA determines immune response to some
pathogens
• African-Americans respond poorly to
interferon-α
• HLA-DR4 associated with high responsiveness
to antigens specific to M tuberculosis but not
to antigens shared with other mycobacteria (p
= 0.0005)
HOST CONTRIBUTES TO
IMMUNOGENICITY
Immunogen dosage
• Dose response
– Not enough to activate lymphocytes or  tolerance
– Too much  tolerance
Example: capsular polysaccharide
– O.5 mg: no IR
– 0.0005 mg: Ab response
IMPORTANCE OF BOOSTERS:  clonal proliferation
HOST CONTRIBUTES TO
IMMUNOGENICITY
Immunogen route of administration
• Routes: experimental Ag via intravenous (IV),
intradermal (ID), subcutaneous (SC),
intramuscular (IM), intraperitoneal (IP)
• Influences which immune organs, tissues and
cells
– IV  spleen
– Subcutaneously  local lymph nodes
HOST CONTRIBUTES TO
IMMUNOGENICITY
Immunogen route of administration
• Routes: vaccine Ag via intramuscular (IM),
oral, subcutaneous (SC),
• Influences which immune organs, tissues and
cells
– SC and IM  local lymph nodes
– Oral  MALT
ROLE OF ADJUVANTS
ADJUVANTS = substances administered with Ag
enhance immunogenicity
Uses: low immunogenicity OR only small amounts
of Ag available
Mechanism of action:
1. Ligands for TLR on DCs and MΦ
2. Prolong Ag persistence “depot effect”
3. Enhance costimulatory molecules
4. Increase local inflammation
5. Stimulate nonspecific lymphocyte stimulation
ADJUVANT TYPES
HUMAN USE
• Alum – aluminum potassium sulfate – APPROVED
• MF59® – water in oil emulsion,  Th1 and Th2 –
APPROVED in EU
• AS04 IN DEVELOPMENT: molecular mechanism based
adjuvants
ANIMAL USE ONLY
• Freund’s incomplete adjuvant
NOT USED EVEN IN ANIMALS
• Freund’s complete adjuvant
EPITOPES
• immune cells do not interact with, or recognize, entire
immunogen molecule
• Lymphocytes recognize discrete sites on immunogen
molecule called epitopes, or antigenic determinants.
• Epitopes are the immunologically active regions of an
immunogen that bind to antigen-specific membrane
receptors on lymphocytes or to secreted antibodies
• Studies with small antigens reveal that B and T cells
recognize different epitopes on same antigenic
molecule
B-CELL EPITOPES
• Hydrophilic amino acids on protein surface
– Protruding regions
– Interior is hydrophobic; must be denatured to be
open to Ab
• These recognize membrane bound or free Ab
B-CELL EPITOPES
B-CELL EPITOPES
• 15 - 22 amino acids on Ag contact Ab
• 75–120 hydrogen bonds as well as by ionic and
hydrophobic interactions.
Conformational Epitope
• Ag-Ab binding due to weak non-covalent
interactions operating over short distances
• Precise complementary shapes increase noncovalent bonding
• Smaller ligands such as carbohydrates, small
oligonucleotides, peptides, and haptens often
bind within deep pocket of Ab
• B-cell epitopes tend to be located in flexible
regions of an immunogen and display site
mobility
• Complex proteins contain multiple
overlapping B-cell epitopes
• Only some are immunodominant
• Determined with monoclonal antibodies
(MAbs)
• More potential antigenic sites than number
recognized by immune system
– Varies from species to species
– Within species, individuals can
• recognize different epitopes as immunogenic and
• mount immune responses that are stronger
(immunodominant) against different epitopes
T CELL EPITOPES
• Antigen processing is required to generate
peptides that interact specifically with MHC
molecules
• Epitope is not conformational rather linear
• Epitopes recognized by T cells are often
internal
• given MHC molecule can selectively bind
variety of different peptides
• Antigen processed into antigenic peptides are
presented in combination with MHC
molecules
• Antigenic peptides recognized by T cells form
trimolecular complexes with T-cell receptor
and MHC molecule
HAPTENS
haptens, small organic
molecules that are
antigenic but not
immunogenic
Become immunogenic
when linked to carrier
molecule, e.g., large
protein
drugs, peptide
hormones, and steroid
hormones
USE OF HAPTENS
• Configuration plays major role in determining
whether it can react with a given antibody
• drugs, peptide hormones, and steroid
hormones