Precipitation of antibody/antigen complexes occurs when there is binding
of soluble Ag and Ab in sufficient amount to come out of solution. There
are three zones which determine the amount of precipitate formed: the zone
of antibody excess, the equivalence zone, and the zone of antigen excess.
In the equivalence zone, the proportion of Ag: Ab is optimal for the
maximum amount of precipitate to form. In the zones of Ab or Ag excess,
the proportions of the reactants do not lead to efficient cross-linking
and therefore do not lead to formation of precipitate.<p>
<address>this information paraphrased from Immunology: a short course, by
Eli Benjamini et al., c 1996 by Wiley-Liss, inc</address>