Avidity is a measure of the synergism of the strength individual interactions when they work as a group. Interactions that are made up of multiple sub-interactions can yield very tight complexes that are relatively easily taken apart and so are frequently used in biology in the formation of transient protein complexes.

For example, the affinities of A binding to C and B binding to C may be greater when A also binds to B. As an example, the interaction of clustered adaptor proteins with the multivalent ligand eps15 is described by the avidity of this interaction. Even though each individual interaction of an eps15 molecule with an adaptor is of relatively low affinity the avidity of eps15 for clustered adaptors is very high.

If the clustered proteins form some sort of matrix (for example a clathrin-coat), the interaction is described by the term matricity. Clathrin is a protein that is the major constituent of the coat of the coated pits and coated vesicles formed during endocytosis of materials at the surface of cells. ...