Heterogeneous ligands fit or 1:1 binding

hi， Dear all
I tested the affinity between an antibody and an antigen, but the fitting results were a bit strange.
For this result, please give some advice。
At 1:1 fitting, the RI value is high，The purity of the antigen& antibody are guaranteed。
When exchanging ligands and analytes, the 1:1 fitting RI is also high。
Can I think of this antibody antigen binding as a two-step reaction？

The first picture shows that the interaction is not fully 1:1. The fitting adds a bulk step to make the fitting 'better', but as you observed this is not right.
The bivalent analyte fitting adds more complexity to the fitting and will always be better.
If the antibody is on the chip you should use the 1:1 interaction; if the antibody is the analyte you can use the bivalent model.
Problem with the two-step model is how to prove it.

Kind regards
Arnoud