Bertrand Friguet Lisa Djavadi Ohaniance and Michel E Goldberg Unit de Biochimie Cellulaire Institut Pasteur Paris France

The affinity (KA) is a thermodynamic parameter that quantifies the strength of the association between two molecules in solution. For antibody-antigen complexes, KA can be interpreted rigorously only for an equilibrium in solution involving a single species of antibody-combining sites and a single species of antigenic sites. Consequently, for polyclonal antibodies which contain a mixture of antibodies, no true affinity can be defined. The affinity of an antibody for an antigen depends on the structural complementarity of the binding site on the antibody (paratope) and the binding site on the antigen (epitope). Antibodies have a wide range of affinities that are generally between 105 (low affinity) and 10i2m 1 (high affinity). The overall stability of the complex between an antibody and an antigen is expressed by the term 'avidity' and is governed by the affinity, the valency of the antibody and the antigen and the steric hindrance of the interacting components. Small changes in antigen structure can affect the strength of the antibody-antigen interaction. Similarly, changing the structure of the binding site of the antibody may alter the affinity of the antibody for the antigen. For instance, the progressive increase in affinity that occurs with time upon immunization (affinity maturation) arises from mutations in the complementarity-determining regions that build the paratope. Thus, antibodies that are different in their binding site structure are expressed and cells producing antibodies with higher affinities are stimulated preferentially to divide.

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