Antibody-antigen complex avidity

If the antigen or antibody of interest is bound to a soEd phase, such as a cell membrane, or to a synthetic particle (polystyrene or ceEulose), the protein wEl exist in a microenvironment that is dffferent from that of a protein in free solution. The water surrounding the protein is more highly ordered near the surface of the solid phase, and a condition results that is more favorable for van der Waals-London dipole-dipole interaction and coulombic bonding. This situation favors the formation of low- and high-avidity antigen antibody complexes and, hence, can provide lower detection limits for analytical applications. Some studies... 

The strength of the antigen-antibody complex is called the affinity or avidity. Affinity refers to the intrinsic association constant between an antibody and a univalent antigen, while avidity refers to the overall binding energy between antibodies and a multivalent antigen. We can write the overall binding reaction as... 

Studies by HeEsing have provided support for the steric exclusion model and have demonstrated that (1) the composition of the immune complex formed is not affected by the presence of a polymer (2) there is no complex formed between the polymer and the antigen, antibody, or immune complex (3) the polymer effect is dependent on the molecular weight of both antigen and polymer (4) and the use of polymer in a reaction mixture can increase the precipitation of immune complex with low-avidity antibody. Addition of polymer to a mixture of antigen and antibody causes a notable increase in the rate of immune complex growth, especially during the early phase of the reaction. ... 

For multivalent interactions (e.g. of antibodies with antigens) the term avidity has been introduced in order to emphasize the stabilization of complexes by... 

The definition of an association constant for an antibody—antigen reaction can become more complex if the antibody—antigen reaction iavolves a multivalent antigen, as is the case when a polyclonal antisemm is used for detection of an antigen. This type of multivalent binding is termed avidity and is defined by the equation ... 

Fig. 8.2. Concept of avidity. Antigens with, e.g., 2 epitopes react with different antibodies of a polyclonal antiserum (I, a and b). However, half-life may often be rather short (few seconds or a fraction of this Section 8.4) resulting in a continuous dissociation-association process. A second antibody (I, c) on the same antigen keeps the complex intact since dissociation from the two antibodies will not occur at exactly the same moment. This afllnity bonus is not obtained with monoclonal antibodies (II), unless two appropriately chosen monoclonal antibodies are mixed (III mixture of monoclonal antibodies to different epitopes does not necessarily lead to an afllnity bonus, e.g. if epitopes are located on opposite sites of the antigen) or if multiple epitopes are present on the antigen (IV).

Scatchard plots can be constructed to estimate the affinity constant r/[antigen] is plotted versus r, where r is the molar ratio of bound antigen to antibody (135,136). For monoclonal antibodies, a straight line is obtained, with the slope equal to K. For polyclonal antibodies, the average K of multiple binding clones can be calculated. However, it will be difficult to assess AT if a complex curvilinear plot is obtained. The same IA method used for sample analysis should be used to test binding avidity. 

Avidity. Avidity is a measure of the overall stability or strength of the antibody-antigen complex. It is controlled by three major factors antibody-epitope affinity the valence of both the antigen and the antibody and the structural arrangement of the interacting ligands. 

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