Binding parameters of antibodies reacting with multivalent antigens: functional affinity or pseudo-affinity.

For antibodies binding bivalently to multivalent antigens, including the cell surface, it has been common to determine the 'functional affinity', even though it is clear that the equations describing true affinity only apply to monovalent interactions. We here summarize evidence that 'functional affinities' are not valid, because they do not accurately describe the interactions that occur. A basic discrepancy is that dissociation, in practice, is generally not a first-order reaction. A more useful approach to compare antibodies is to determine the kinetics of dissociation over a long time period, until near-complete dissociation has occurred.