Enzymes labelled conjugates were first introduced in 1966 for localization of antigens in tissues, as an alternative to fluorescent conjugates.
In 1971, enzyme labelled antigens and antibodies were developed as serological reagents for the assay of antibodies and antigens.
The term enzyme immunoassay (EIA) includes all assays based on the measurement of enzyme labelled antigen, hapten or antibody.
EIAs are of two basic types – homogenous and heterogeneous.
In homogeneous EIA, there is no need to separate the bound and free fractions so that the test can be completed in one step, with all reagents added simultaneously. This type of EIA can be used only for assay of haptens such as drugs and not for microbial antigens and antibodies.
Heterogeneous EIA requires the separation of the free and bound fractions either by centrifugation or by absorption on solid surface and washing. It is therefore a multi step procedure, with reagents added sequentially. The major type of heterogeneous EIA is enzyme linked immunosorbent assay (ELISA).
ReferenceText Book Of Microbiology