Avidity

In proteins, avidity is a term used to describe the combined strength of multiple bond interactions. Avidity is distinct from affinity, which is a term used to describe the strength of a single bond. As such, avidity is the combined synergistic strength of bond affinities rather than the sum of bonds.

It is commonly applied to antibody interactions in which multiple antigen binding sites simultaneously interact with a target. 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 site is likely to be reinstated. The overall effect is synergistic, strong binding of antigen to antibody (e.g. IgM is said to have low affinity but high avidity because it has 10 weak binding sites as opposed to the 2 strong binding sites of IgG, IgE and IgD).

If the clustered proteins form a matrix, such as a clathrin-coat, the interaction is described by the term matricity.