In am ELISA you have the solid phase antibody and the laballed antibody in excess in the reactions. Set this into the law of mass actions (reaction step by reaction step) and you will see that the affinity determins the detection limit because the antigen is the limiting part in this reaction. 

You wiil se also, that the affinity of the first reaction is much more important than the affinity in the second reaction.

The is a simple method to determine the affinity by ELISA, described by Friquet: http://www.ncbi.nlm.nih.gov/pubmed/3981007

cave: If you are using the same antibody in both reactiony, you have to make sure that the respective epitope is at least twice present in your antigen.

Dear Steephan T. Kiessig.

Thanks a lot  for your answer. In this case, I am working with capsular polisacharides so epitopes are multi represented on the molecules.

Thanks again

than it's should work. For the affinity detection you need to have a defined epitope, may be that you can use a tri-, tetra or pantasaccharide.