more than one of an antibody’s binding sites, the binding strength is referred to as avidity. Where
multivalent bonding is involved the avidity is orders of magnitude greater than the sum of the
affinities for each antigenic determinant and each antibody binding site. Avidity is also relevant to35
antibodies bound to a substrate, such as used in immunometric assays. The close proximity of the
immobilized antibodies to each other on the carrier surface tends to give an avidity or functional
affinity which is substantially higher than the affinity constant for the free antibody.36
Specificity is another important property related to antibodies. Specificity is the ability of an
antibody to discriminate between the antigen against which it was made (the immunogen) and any
other antigen. The converse of specificity is cross-reactivity, the tendency of an antibody to interact37
with antigens other than its immunogen. Generally, the affinity constant for a cross-reaction is38
lower than the affinity constant for the reaction between an antibody and its immunogen.39
Immunoassays are diagnostic techniques which utilize the specificity and reactivity of antigens
and antibodies to detect or quantitate antigens or antibodies in a solution. Immunometric or40
"sandwich" assays of the type set forth in the count are used to determine the presence and/or
concentration of a particular antigen in a fluid. One type of immunometric assay uses a solid41
insoluble substrate or carrier having antibodies for a particular antigen bound to the carrier surface.
A fluid thought to contain the antigen of interest is contacted with the carrier. If present, the antigen
reacts with the carrier-bound antibody forming a complex. Free labeled antibody which also reacts
with the antigen is added, simultaneously or subsequently, to the solution. The free labeled antibody
reacts with the bound antigen. This two-part reaction results in an insoluble three-part complex in
the form of a sandwich. The substrate-bound and the labeled antibodies form the "bread" while the
35
IMMU, p. 6.3.
36
FUND, p. 432-33.
37
FUND, p. 440.
38
FUND, p. 440.
39
FUND, p. 440.
40
FUND, pp. 433-440.
41
Engvall specification, p. 4.
7
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