Tutorial for Titrations: page 2
Antibody titrations are not always as clear as one would expect! Dilutions do not always shift the fluorescence histogram peak in the direction expected, as one would normally assume that at higher concentrations of the antibody, the histogram peak would further to the right on a single parameter frequency distribution . If you look at the histogram display below you will see a nice example of the problem that can be occassionally encountered.
This affect is known as the Prozone phenomenon and occurs when there is a situation of antibody excess. Thus, by titrating the antibody further out, the antibody concentration decreases to the point where antibody and antigen are in equivalence and maximum binding can occur.
For further reading on this topic see Ch 26. "Clinical Laboratory Methods for Detection of Antigens and Antibodies" in "Basic and Clinical Immunology 3rd edition" Fundenberg et al. LANGE Medical Publications.