The bracketed terms, [Ab*Ag], [Ab] and [Ag], represent the equilibrium concentrations in moles27
per liter of the antibody/antigen complex, the antibody and the antigen, respectively. The “•”28
indicates the mathematical product of the adjacent terms. The units of affinity are liters/mole. The29
calculated value of the affinity constant is a characteristic of the specific antibody/antigen reaction,
rather than a characteristic of a particular antibody.30
As can be seen from the above equation, the mathematical maximum and minimum
possibilities for affinity are 4 and 0. Where all antibody and antigen bind to form Ab*Ag and remain
bound (i.e., the binding is so strong that there is no reverse reaction), the concentration of antibody
31
would be zero giving an affinity constant of 4. If no reaction takes place, there would be no
32, 33
concentration of product Ab*Ag, giving an affinity value of 0.
In reality, affinity falls between these two values. All antibody/antigen reactions have an
affinity constant. The higher the affinity constant value, the tighter the binding of the antibody and
the antigen and the lower the concentration of the antigen that can be detected. The subject matter34
of this interference relates only to antibodies having an affinity constant of about 10 liters/mole and8
higher.
Avidity is a concept related to affinity. Because antibodies have two binding sites, antibodies
are potentially multivalent in the reaction with antigen. Antigens may also have more than one
antigenic determinant and may also be multivalent. When such a multivalent antigen combines with
27 23
A mole is the weight in grams of 6.02 X 10 molecules of a chemical. CHEM, p. 45.
28
IMMU, p. 6.6
29
[moles/liter] = 1 = liters/mole
[moles/liter][moles/liter] [moles/liter]
30
Throughout this opinion, the use of the word “affinity” means the affinity constant of the relevant
antibody/antigen reaction.
31
K = [Ab*Ag] = 4
[0]·[0]
32
K = [0] = 0
[Ab][Ag]
33
It is to be recognized that “0" and “4” are theoretical mathematical limits and actually represent “non-
reversible reactions.”
34
IMMU, p. 6.6.
6
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