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Lecture 14 Antibody-Antigen Reactions

Lecture 14 Antibody-Antigen Reactions. POOR FIT. high repulsion. high attraction. low attraction. low repulsion. Binding of the epitope in the antigen binding site. GOOD FIT. antibody combining site. antigen determinant. Antibody Avidity.

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Lecture 14 Antibody-Antigen Reactions

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  1. Lecture 14Antibody-Antigen Reactions

  2. POOR FIT high repulsion high attraction low attraction low repulsion Binding of the epitope in the antigen binding site GOOD FIT antibody combining site antigen determinant

  3. Antibody Avidity • Multiple interactions between antigen binding sites and epitopes • Greater than additive • Similar to Velcro™ • More relevant to biologic systems than affinity

  4. Effect of multivalent interactions antibody Fab IgG IgG IgM effective antibody valence 1 1 2 up to 10 antigen valence 1 1 n n equilibrium constant (L/M) 10 10 10 10 4 4 7 11 advantage of multi-valence - - 10 -fold 10 -fold 3 7 definition of bindng affinity affinity avidity avidity intrinsic affinity functional affinity

  5. Biological Consequences of Antibody Affinity/Avidity • Neutralization of toxins • Complement activation • Immune elimination of antigen • Virus neutralization • More intense immune complex disease in animals • higher levels of circulating antigen-antibody complexes • more intense localization of immune complexes on basement membranes. • more severe impairment of organ function

  6. Cross-Reaction A B C D Immunize Immunogen anti-B anti-C anti-D anti-A Reactivity with rabbit antiserum A B C D A E F G Cross-reactive antigen

  7. Primary Interactions Between Antigen and Antibody • Determined by the interaction between epitopes and antigen-binding sites • Independent of biochemical and biologic phenomena that may follow • Depend only on the quantity and quality (affinity/avidity) of antibodies • The basis for ELISA, RIA, IFA assays

  8. Enzyme (ELISA) Immunoassay Enzyme conjugated to anti-Ig antibody (“second antibody”) Add substrate for enzyme anti-antigen antibody Antigen

  9. Immune Precipitation Antigen Antibody

  10. Radial Immunodiffusion Ab-containing gel Precipitin ring Ag/4 Ag/2 Ag/8 Ag/1 D2 Ring Antigen Concentration

  11. RBC RBC RBC RBC RBC RBC RBC RBC RBC RBC RBC Agglutination IgM Antibody IgG Antibody

  12. Complement activation • A system of plasma proteins that interact with • Antigen/antibody complexes • Pathogen surface motifs (alternative and lectin pathways • Activation of complement results in • Chemo-attraction of inflammatory cells • Peptide mediators of inflammation (anaphylatoxins) • Increased blood vessel permeability • Smooth muscle contraction • Mast cell degranulation • Opsonization of pathogens (enhances phagocytosis) • Killing of pathogens (membrane attack complex)

  13. Overview of the Complement Cascade

  14. Effector Actions of Complement

  15. Affinity Agglutination Avidity Chemotaxis Complement Classical pathway Alternative (properdin) pathway Cross-reaction Epitope/Antigenic Determinant Opsonization Precipitation Second antibody Specificity Valence Terms to remember

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