The binding strength between a single antibody and a single antigenic determinant is measured
by the affinity constant for the antibody/antigen interaction. The affinity constant is the association19
or equilibrium constant for the antibody/antigen reaction. The magnitude of the affinity constant20
indicates the reactivity of an antibody with the antigen and the stability of the complex formed.
Because the concept of the affinity constant is important to our decision we describe the concept in
some detail.
The affinity constant is the equilibrium constant for the reaction of the antibody with the
21 22
antigen of interest. Such reactions are reversible. Reversible reactions do not stop but reach a
dynamic equilibrium in which the reaction rate in the forward and reverse directions are equal. The 23
reversible reaction of an antibody with the specific antigen may be represented by the following
equation:24
Ab % Ag º Ab( Ag .
Ab represents the antibody, Ag represents the antigen and Ab*Ag represents the antibody-antigen
reaction product or complex. The affinity constant for the reaction is the equilibrium constant, K,25
defined by the following equation: 26
K ' [Ab( Ag]
[Ab] • [Ag]
.
19
IMMU, p. 6.3; 6.6; glossary.
20
FUND, p. 422.
21
IMMU, P. 6.3.
22
IMMU, p. 6.3; CELL, p. 1007; FUND, p. 422.
23
PHYCHEM, pp. 168-169.
24
IMMU, p. 6.3.
25
IMMU, p. 6.3.
26
IMMU, p. 6.3.
5
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