Zone of Equivalence
Definition
The concentration range at which antigen and antibody molecules are present in the correct ratio for maximum lattice formation and visible precipitation. Outside this zone, in antigen or antibody excess, complexes remain soluble and no precipitate forms, giving a false-negative appearance.
- Context
- Antigen-antibody precipitin reactions
- Also called
- Zone of equivalence / equivalence point
- Outside the zone
- Antigen or antibody excess
- Failure mode
- Soluble complexes give a false-negative appearance
Common questions
How do serologists avoid a false negative caused by antigen excess?+
Serial dilution of the questioned extract is tested against a fixed antiserum concentration, so at least one dilution should fall inside the equivalence zone and produce a visible precipitin line even if the neat extract does not.
Does the zone of equivalence differ between test formats?+
Yes. Its practical width depends on the reagents and format used, so a double-diffusion (Ouchterlony) plate and a ring test can show equivalence at different relative concentrations even for the same antiserum.
Related terms
- Antiserum
- Serum collected from an immunised animal containing antibodies directed against a specific antigen or group of antigens. In species identification, antiserum raised...
- Counter-Immunoelectrophoresis (CIE)
- An accelerated form of gel diffusion in which an electric current drives antigen and antibody toward each other rather than relying on...
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Ouchterlony Double Diffusion
- A gel diffusion technique in which antigen and antibody placed in separate wells in an agar gel diffuse passively toward each other....
- Precipitin Reaction
- The formation of an insoluble antigen-antibody complex (precipitate) when soluble antigen and antibody meet at equivalent concentrations. The Ouchterlony double-diffusion test and...