What is the relationship between Kd and antibody affinity?

Kd and antibody affinity have an inversely proportional relationship. The Kd value represents a specific type of equilibrium constant for the separation of the antibody-antigen complex into individual, smaller components. Antibody affinity refers to the binding strength between an antibody and antigen. Thus, the Kd value can be used to measure the binding affinity. The formula for calculating Kd (for antibody affinity) is: 

Kd=([Antibody][Antigen])/[Antibody-Antigen Complex]

The antibody and antigen represents free concentrations of each molecule, and the antibody-antigen complex refers to concentration of the antibody-antigen complex (at equilibrium). This equation comes from the equilibrium dissociation constant equation which is: 

([Ax][By])/[AxBy]

The variables (antibody, antigen), and antibody-antigen complex are used for the components representing Ax, By, and AxBy, respectively. A lower Kd value indicates a higher affinity of the antibody, while in contrast a higher Kd value indicates a lower affinity of the antibody.