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Lecture 3 Antibody-Antigen Reactions Binding of the epitope in the antigen binding site GOOD FIT POOR FIT antibody combining site antigen determinant high attraction low repulsion high repulsion low attraction Antibody Avidity Multiple interactions between antigen binding sites and epitopes Greater than additive Similar to Velcro™ More relevant to biologic systems than affinity 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 4 10 4 10 7 10 11 advantage of multi-valence - - 10 3 -fold 10 7 -fold affinity affinity avidity avidity definition of bindng intrinsic affinity functional affinity 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 Cross-Reaction A B C D Immunize Immunogen anti-A A B C D A E F G Cross-reactive antigen anti-B anti-C anti-D Reactivity with rabbit antiserum Antigen-Antibody Interactions Quality and quantity are important in resolution of disease May contribute to pathology Useful in immunological assays Immune Precipitation Antigen Antibody Agglutination RBC IgM Antibody RBC RBC RBC RBC RBC RBC RBC IgG Antibody RBC RBC RBC Enzyme (ELISA) Immunoassay Enzyme conjugated to anti-Ig antibody (“second antibody”) anti-antigen antibody Antigen Add substrate for enzyme 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) Overview of the Complement Cascade Effector Actions of Complement Terms to remember Affinity Agglutination Avidity Complement Classical pathway Alternative (properdin) pathway Cross-reaction Epitope/Antigenic Determinant Precipitation Second antibody Specificity Valence