Like antibodies, antigens can be multivalent, either through multiple copies of the same epitope, or through the presence of multiple epitopes which are recognized by multiple antibodies. Interactions involving multivalency can produce more stabilized complexes, however multivalency can also result in steric difficulties, thus reducing the possibility for binding. The time taken to reach equilibrium is dependent on the rate of diffusion and the affinity of the antibody for the antigen, and can vary widely. Affinity constants can be affected by temperature, pH and solvent. Affinity constants can be determined for monoclonal antibodies, but not for polyclonal antibodies, as multiple bondings take place between polyclonal antibodies and their antigens.
Avidity is a measure of the overall stability of the antibody-antigen complex.
It is controlled by three major factors - the affinity of the antibody for the epitope, the valency of the antigen and antibody, and the structural arrangement of the interacting parts. Immunochemical techniques capitalize upon the extreme specificity, at the molecular level, of each immunoglobulin for its antigen, even in the presence of high levels of contaminating molecules. The multivalancy of most antigens and antibodies enables them to interact to form a precipitate. When designing procedures, it is important to differentiate between monoclonal and polyclonal antibodies, as these differences are the foundation of both advantages and limitations for their use. Many of the antibodies used in immunochemical techniques are raised by repeated immunization of a suitable animal, e.g. rabbit, goat, donkey, or sheep, with a suspension of the appropriate antigen.