Dissociation Constant (Kd)

Definition

Mathematical representation of the strength of binding between the antigen-binding site and the antigen (affinity)

$$ {{\text{K}}_{\text{d}}} = {{\left[ {\text{Ab}} \right]\left[ {\text{An}} \right]} \mathord{\left/ {\vphantom {{\left[ {\text{Ab}} \right]\left[ {\text{An}} \right]} {\left[ {\text{C}} \right]}}} \right. } {\left[ {\text{C}} \right]}} $$

where [Ab] is the concentration of unbound antibody, [An] is the concentration of unbound antigen, and [C] is the concentration of the Ab-An complex. The smaller the K_d, the more tightly bound is the Ab-An complex and higher the affinity. The association constant (K_a) may also be used and is the inverse of K_d.

Cross-References

Targeted Radioimmunotherapy