Equivalence Zone
Definition
The range of antigen-to-antibody ratios at which binding sites on both molecules are maximally cross-linked, producing the largest lattice and the heaviest precipitate. Deviation in either direction yields smaller, soluble complexes and reduced or absent precipitation.
- Also called
- Zone of equivalence, optimal proportion
- Result at equivalence
- Maximum lattice, heaviest precipitate
- Deviation effect
- Smaller soluble complexes, reduced precipitation
- Used in
- Precipitin and precipitation tests
Common questions
Why might a precipitation test show no visible precipitate even though both antigen and antibody are present?+
If either reagent is in large excess relative to the other, the ratio falls outside the equivalence zone and only small, soluble complexes form instead of the large cross-linked lattice needed for visible precipitation, a phenomenon called the prozone or postzone effect.
How do analysts locate the equivalence zone in practice?+
By testing a dilution series of antigen against a fixed antibody concentration, or the reverse, and identifying the dilution that produces the heaviest precipitate, which marks the point of maximal cross-linking.
Related terms
- Double Immunodiffusion (Ouchterlony)
- A gel diffusion method in which both antigen and antibody diffuse toward each other from separate wells in agar. Precipitin bands form...
- Immunoelectrophoresis
- A two-step technique: proteins are first separated by electrophoresis in agar gel, then antiserum diffuses from a parallel trough and forms precipitin...
- Lattice Theory
- The structural explanation for precipitation proposed by Marrack in 1934. Multivalent antigens and bivalent antibodies can each bind more than one partner,...
- Precipitin
- An antibody that forms a visible precipitate when it reacts with its specific soluble antigen. The term is used interchangeably with precipitating...
- Ring Precipitation Test
- A technique in which antiserum is placed in a narrow capillary tube and antigen solution is layered carefully on top. A ring...