[citation needed] Binding affinity is a measure of dynamic equilibrium of the ratio of on-rate (kon) and off-rate (koff) under specific concentrations of reactants.
[citation needed] There are several methods to investigate protein–protein interactions existing with differences in immobilization of each reactant in 2D or 3D orientation.
[citation needed] As an example, affinity is the binding strength between the complex structures of the epitope of antigenic determinant and paratope of antigen-binding site of an antibody.
[3] Calculation of binding affinity for bimolecular reaction (1 antibody binding site per 1 antigen): where [Ab] is the antibody concentration and [Ag] is the antigen concentration, either in free ([Ab],[Ag]) or bound ([AbAg]) state.
[4] A number of technologies exist to characterise the avidity of molecular interactions including switchSENSE and surface plasmon resonance.