Precipitation Curve (Prozone and Postzone)
- On either side of the equivalence zone, precipitation is actually prevented because of an excess of either antigen or antibody. In the case of antibody excess, the prozone phenomenon occurs, in which antigen combines with only one or two antibody molecules, and so no cross-linkages are formed. At the other side of the zone, where there is antigen excess, the postzone phenomenon occurs, in which small aggregates are surrounded by excess antigen, and again no lattice network is formed.
Zone of Equivalence
- In addition to the affinity and avidity of the antibody involved, precipitation depends on the relative proportions of antigen and antibody present. The zone in which optimum precipitation occurs is called the zone of equivalence, in which the number of multivalent sites of antigen and antibody are approximately equal. Precipitation is the result of antibody binding to more than one antigen and vice versa, forming a stable network or lattice. In this case each antibody must have at least two binding sites and antigen must be multivalent. As they combine this results in a multimolecular lattice that increases in size until precipitate out of solution.
Precipitation Curve
