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avid·i·ty (ə-vidґĭ-te) 1. the strength of an acid or a base. 2. the strength of binding between antibody and a complex antigen. Since the antigen has more than one determinant and many of the determinants differ from one another, avidity expresses the overall interaction between antigen and antibody; it is, however, greater than the sum of the affinities for the single determinants, since the effective multivalency of the antigen gives rise to a cooperative “bonus†effect. Often represented by constant Ka (the value of the association constant for the reaction Ab + Ag ⇄ AbAg). Avidity is a function of the techniques used in its measurement and can be expressed only in arbitrary units. Cf. affinity.Medical dictionary. 2011.