**9.3 Affinity and avidity**

The antibody affinity is a measure of the strength of binding of an individual antibody binding site to a single antigenic site. This can be considered as the sum of all the non-covalent interactions between antibody and antigen involved in the binding reaction. However, antibody molecules usually have more than one binding site and many antigens contain more than one antigenic site and therefore multivalent binding may be possible. The strength with which a multivalent antibody binds to an antigen, is termed avidity. Although high affinity is a requirement for good tumor localization, there seems to be a point at which further increases in affinity do not increase uptake at the target site (Schlomet al., 1992; Colcher et al., 1988). Indeed, reduced tumor uptake and limitations on penetration of antibody into the tumor tissue can result from the increasing of antibody affinity (Dearling & Pedley, 2007).
