Immunogens and Antigens *
Jeffrey K. Actor, Ph.D.
Pathology and Laboratory Medicine
The University of Texas-Houston Medical School
* Special thanks to Dr. L. Scott Rodkey, Ph.D.
Immunogen vs. Antigen
Immunogen-Agent capable of inducing an immune
response
Antigen-Agent that binds specifically to preformed
antibodies or T cells
Requirements for Immunogenicity
The B cell Receptor (Antibody)
• Foreignness-Guideline, not requirement
• Molecular weight-Compounds >6,000 are
usually immunogenic.
• Chemical Complexity
• Degradability
Requirements for Immunogenicity
The T cell Receptor (TCR)
T cells must have antigen
presented to them!
• T cell antigens require presentation by the
Major Histocompatibility molecules present on
antigen presenting cells.
• Antigens are limited to small peptides.
• In Humans, MHC is called HLA (Human
Leukocyte Antigens).
Hapten
• Low Molecular Weight
• Non-Immunogenic Alone
• Coupling to immunogenic compound
(protein carrier) renders it immunogenic.
• Can bind pre-formed Antibodies.
Hapten coupling and injection
Injecting hapten alone does not confer antibody response.
Only haptens coupled to larger molecules will elicit response.
Clinical: Haptens in Pediatrics
• Wyeth makes HibTITER—H. influenza b capsular
oliogosaccharide coupled to diphtheria CRM197
(non-toxic variant). They also make a
pneumococcal vaccine PREVNAR with same
carrier.
• Merck makes similar vaccine of H. influenza b
coupled to outer membrane protein complex of
Neisseria meningitidis serogroup B.
• In each case, the hapten is rendered T-dependent.
Epitopes
• Site on antigen surface recognized by
antibody paratope.
• Upper size is 7 X 12 X 35 Angstroms.
• Lower size is undefined
Major Antigen Classes
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Proteins, glycoproteins - Most common
Carbohydrates, polysaccharides
Lipids
Nucleic Acids
Miscellaneous-Metals (Ni, Hg), organic
chemicals
, glycolipid
Sequential and Conformational
Epitopes
• Sequential Epitopes-Short stretches of
amino acids (4-7) recognized by Ab when
free in solution, Ag is denatured, or native.
• Conformational Epitopes-Require the
unaltered, native 3-D shape of Ag for
recognition by Ab.
Sequential and Conformational
Epitope Examples
Physico-Chemical Forces
Involved in Ag-Ab Interactions
• Electrostatic Interactions-Attraction of a + charge
to a – charge (Glu to Lys).
• Hydrogen Bonds-Bridging of a COO- to the H of
an OH group.
• Hydrophobic Bonds-Interaction of hydrophobic
–R groups of Ag and Ab.
• Van der Waal’s Forces-Attraction of oscillating
dipoles, weak, 7th power of distance
Noncovalent Forces contribute to strength of AntibodyAntigen interactions.
Major Antigen (Ag) Classes
Carbohydrates, Polysaccharides
Proteins, Glycoproteins
Nucleic Acids
Lipids
Electrostatic
interactions
Ag
Ag
Hydrogen
bonding
Hydrophobic
bonding
Van der Waal’s
forces
Requirements for Immunogenicity
Physiochemical Complexity
Molecular Weight >6 kD
Foreignness (non-self)
Degradability
Immunological Specificity
Refers to the ability of a polyclonal antiserum
to distinguish minor structural differences
between the original immunogen or hapten
and similar, yet structurally different
antigens or haptens.
Cross-reactivity
• Antibody can react with two similar, yet
different antigens or haptens.
• The two may share one or more identical
epitopes.
• The two may share one or more structurally
similar, yet different epitopes.
Specificity Example
Cross-reactivity Examples
1. Toxoids-Diphtheria, Tetanus
2. Blood Group Antibodies
Adjuvants
• Adjuvants are substances that are nonimmunogenic alone but enhance the
immunogenicity of any added immunogen.
Examples of Adjuvants
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Freund’s Complete Adjuvant
Lipopolysaccharide
Muramyldipeptide from TB
Synthetic Polynucleotides
Aluminum Hydroxide (Alum)-The only adjuvant with
FDA approval for human use, used in DT and Tetanus
vaccines.
• Cytokines
• MF59-Europe, not US. Squalene + 2 surfactants.
Coda
• Successful immunization depends on HLA
haplotype of patient, dose of antigen, route
of immunization, use of adjuvant, immune
status of patient, existing allergies,
heterogeneity of antigen mixture, physical
state of antigen, stress level of patient, etc.
etc. etc.
Example of Ag-Ab Bonding