Antigen-antibody interactions: Antigen-antibody interactions

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:

• 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)

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