This technology measures binding events in solution without labeling of the binding partners.  A wide range of binding constants (K_d) can be measured down to 10^-13 M. On and off rates as fast as 10⁹ M^-1s^_-1 and 10^-7, respectively, can reportedly be measured. KinExA systems have been compared to surface plasmon resonance (SPR) determinations and a comparison of antibody binding measuremenets via four biosensor (Biocore, ProteOn, Octet, and KinExA) has been done by Rinat Laboratories (Pfizer).  The technology is based upon kinetically excluding the dissociation of a complex. For a sample binding pair, antibody (A) and antigen (B), B is first bound to a solid phase in excess of the concentration of A to be measured. A mixture of A, B, and AB complex is then briefly exposed to the solid phase. Because the exposure is short and immobilzed L is in excess, no dissociation of the complexed AB is observed and only the free (uncomplexed) A is captured. The amount of A bound can be measured with a sedondary fluorescent label specific for A. The fluorescence signal from the captured A is linearly related to the concentration of free A in the solution phase allowing K_d calculations. 

Applications:

1. Antibody engineering
2. Drug discovery
3. Environmental contaminants

Pros:

1. No immobilization of the sample
2. No labeling of either binding partner needed

Cons:

1. Best for stable interactions
2. Does require immobilization of ligand