Chemical affinity

In biochemistry, avidity refers to the accumulated strength of multiple affinities of individual non-covalent binding interactions, such as between a protein receptor and its ligand, and is commonly referred to as functional affinity. Avidity differs from affinity, which describes the strength of a single interaction. However, because individual binding events increase the likelihood of occurrence of other interactions (i.e., increase the local concentration of each binding partner in proximity to the binding site), avidity should not be thought of as the mere sum of its constituent affinities but as the combined effect of all affinities participating in the biomolecular interaction. A particular important aspect relates to the phenomenon of 'avidity entropy'. Biomolecules often form heterogenous complexes or homogeneous oligomers and multimers or polymers. If clustered proteins form an organized matrix, such as the clathrin-coat, the interaction is described as a matricity.

==Antibody-antigen interaction== Avidity is commonly applied to antibody interactions in which multiple antigen-binding sites simultaneously interact with the target antigenic epitopes, often in multimerized structures. Individually, each binding interaction may be readily broken; however, when many binding interactions are present at the same time, transient unbinding of a single site does not allow the molecule to diffuse away, and binding of that weak interaction is likely to be restored.