Specificity titer and affinity

Several parameters have to be evaluated to establish the quality and usefulness of an antiserum. These are specificity, titer and affinity.

The most outstanding feature of a good antiserum is specificity. Most antigens express a large number of epitopes and, against each of these, different antibodies may be elicited. A highly specific antiserum is one that reacts only with epitopes on the antigen used to raise it and not to other antigens that resemble it. In practice an antiserum reacts with antigens that in one way or another resemble the immunizing antigen. Extensive absorption against these cross-reacting antigens will yield a monospecific antiserum.

A high titer antiserum is one which has a large number of antibodies that are specific for the antigen being tested. The titer of an antiserum is generally expressed as the reciprocal of the highest dilution that reacts with antigen to a predetermined end-point in a given assay system.

The affinity of an antibody reflects the strength of binding between a single antibody-combining site (paratope) and an antigenic determinant (epitope). The term avidity is thus used to describe the overall strength of binding between the antigen and antibody. Avidity is influenced by the intrinsic affinity of the antibody for an epitope on its antigen and the valency of both the antibody and the antigen. As a rule, when an antibody and antigen can form multivalent complexes, the overall strength of the interaction (avidity) is greatly increased.

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