Low affinity antibodies bind antigen weakly and tend to dissociate readily. High affinity antibodies bind antigen more tightly and tend to remain bound longer. The association constant, K, is a measure of affinity. Thus, the affinity at one binding site does not necessarily reflect the true strength of the interaction between the whole antigen and the whole antibody molecule. The total strength of all interactions between multivalent antigen and antibody is called the avidity of the antibody.
Avidity can compensate for low affinity.
For example, an IgM antibody may have lower affinity for a particular antigen than an IgG molecule but may have higher avidity because of its multiple binding sites. This may occur if two different antigens share an identical epitope or if a second epitope possesses similar chemical properties to the first. Usually the antibody's affinity for the cross-reacting antigen will be lower than its affinity for the eliciting antigen. Polyclonal antibodies Immunization of a suitable animal with a suitable antigen in a suitable manner leads to production of antibodies directed against epitopes of that antigen by the B cells of the animal. A given B cell with a specificity for an epitope of the immunizing antigen undergoes proliferation to give rise to a large number of progeny, all of which produce antibodies directed against the same epitope.