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 CRM 197 (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 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, 7 th power of distance
Noncovalent Forces contribute to strength of Antibody- Antigen interactions. Electrostatic interactions Major Antigen (Ag) Classes Carbohydrates, Polysaccharides Proteins, Glycoproteins Nucleic Acids Lipids Hydrophobic bonding Ag Ag Hydrogen 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 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