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 - 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
AGGLUTINATION AGGLUTINATION 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: Agglutination/Hemagglutination: 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)
NEUTRALIZATION NEUTRALIZATION 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
OPSONIZATION OPSONIZATION 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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