EIA is one of a group of binding assays in which the
molecular recognition properties of antibodies are used.
For high sensitivity these immunoassays usually make
use of antigen, hapten, or antibody labeled in some way.
The most commonly used label is a radioisotope, and
this has given rise to the different forms of radioimmunoassay
(RIA) (1). Various non-isotopic labels have
also been investigated and this has allowed the development
of other types of immunoassay. The following
labels have been used: an enzyme (EIA) (2), an erythrocyte
(hemagglutination inhibition) (3), a bacteriophage
(viroimmunoassay) (4), a fluorescent group
(fluoroimmunoassay) (5), and a stable free radical (spin
immunoassay) (6). In all these immunoassays the labeled
component of an antibody/antigen reaction binds
to its complementary binding site. The amount bound
depends on the concentrations of the other components
of the system and if one of these is varied this causes a
change in the distribution of the labeled component
between the bound and unbound fractions. The properties
of the label are used to determine its distribution,
and thus a calibration curve can be constructed relating
concentration of the label to the concentration of the
component varied.