**11.11 Antibody affinity**

The strength of the antibody:antigen interaction is measured through the binding affinity and is quantified through the association constant (Ka) (Boswell et al., 2007). Increasing Ka of antibody will be leads to an increase in uptake antibody in the tumor. However, they may be less able to penetrate deeply due to strongly bound at the tumor surface. The studies with iodine radiolabeled scFvs demonstrated that a threshold affinity between 10−7 and 10−8 M was required to observe detectable tumor uptake in mice 24 h post injection, whereas no gain in tumor accumulation was observed with affinities >10−9 M. Affinities >10−9 M were detrimental to rapid and uniform tumor penetration due to stable binding at the first pass of tumor antigens forming a binding site barrier (Adams et al., 2001).
