Antigen-antibody interactions: Antigen-antibody interactions

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Antigen-antibody interactions:

  • Antigen-antibody interactions:

  • Are reversible specific non-covalent biochemical reactions:

  • Hydrogen bonds (A chemical bond in which a hydrogen atom of one molecule is attracted to an electronegative atom of another molecule)

  • Electrostatic bonds(A valence bond in which two atoms, attracted by electrostatic forces, transfer one or more electrons between atoms)

  • Van der Waal forces (forces acting between nonbonded atoms or molecules)

  • Hydrophobic bonds(The attractive force between molecules due to the close positioning of non-hydrophilic portions of the two molecules

  • Can be represented by the formula:

  • K1=constant of association

  • K2=constant of dissociation

The affinity:

    • The affinity:
    • is the strength of the reaction between a single antigenic determinant and a single combining site on the antibody
    • or it is the association constant for binding (KA)
    • KA= k1/k2
    • Valence: the number of epitopes
    • Avidity: is the collective affinity of multiple binding sites(affinity+ Valence)


    • Precipitation
    • Agglutination
    • Neutralization (Antitoxins)
    • Opsonization
    • Antibody-dependant cell-mediated cytotoxicity
    • The complement activation Membrane attack complex

Is the reaction of soluble Ag with soluble Ab.

  • Is the reaction of soluble Ag with soluble Ab.

  • The reaction results in the formation of Ag-Ab complexes (lattices)

The Quantitative Precipitation Reaction:

  • The Quantitative Precipitation Reaction:

  • Varying amounts of Ag are mixed and incubated with Constant volume of antisera

  • Precipitate is measured, amount of precipitate depends on :

    • the ratio of Ag : Ab
    • The Ab avidity
  • Plot in a curve, three zones are detected:

  • Zone of Ag excess : insufficient Ab  too small complexes to precipitate

  • Equivalence zone : large lattice is formed  visible precipitates

  • Zone of Ab excess : not enough Ag  too small complexes to precipitate



  • Abs can bind and cross-link cells or particles  aggregate formation

  • Entrap microbial invaders

  • IgM & IgA are the most suitable (IgG in sufficient amounts can agglutinate cells)


  • Agglutination/Hemagglutination:

  • a. Qualitative agglutination test

  • Determination of blood types or antibodies to blood group Ags

  • b. Quantitative agglutination test

  • Agglutination tests can also be used to measure the level of antibodies to particulate antigens.(titration)

  • Passive hemagglutination: erythrocytes are coated with a soluble antigen (e.g. viral antigen, a polysaccharide or a hapten) and use the coated red blood cells in an agglutination test for antibody to the soluble antigen

  • Coomb's Test (Antiglobulin Test)



  • Is the binding of Ab to microbial epitopes or soluble molecules(e.g. toxins) which inhibits their binding to host cells.

  • Abs are mostly IgG & IgA

    • Used to identify toxins and viruses



  • Is the process by which a pathogen is marked (tagged) for ingestion and destruction by phagocytic cells

Antibody-dependant cell-mediated cytotoxicity

    • Antibody-dependant cell-mediated cytotoxicity
  • Coating of an organism can attract phagocytic cells as well as other cytolytic cells(NK cells, eosinophils)

  • The organism may be: bacteria, protozoa, parasitic worms

  • These cells use cytolytic mechanisms to kill those organisms

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