 For example, IgM is said to have low affinity but high avidity because it has 10 weak binding sites for antigen as opposed to the 2 stronger binding sites of IgG, IgE and IgD with higher single binding affinities.Binding affinity is a measure of dynamic equilibrium of the ratio of on-rate (kon) and off-rate (koff) under specific concentrations of reactants.The affinity constant, Ka, is the inverse of the dissociation constant, Kd.