Indirect ELISA
For indirect detection, the antigen coated to a multi-well plate is detected in two stages or layers. First an unlabeled primary antibody, which is specific for the antigen, is applied. Next, an enzyme-labeled secondary antibody is bound to the first antibody. The secondary antibody is usually an anti-species antibody and is often polyclonal. The indirect assay, the most popular format for ELISA, has the advantages and disadvantages:
Advantages
A wide variety of labeled secondary antibodies are available commercially.
Versatile because many primary antibodies can be made in one species and the same labeled secondary antibody can be used for detection.
Maximum immunoreactivity of the primary antibody is retained because it is not labeled.
Sensitivity is increased because each primary antibody contains several epitopes that can be bound by the labeled secondary antibody, allowing for signal amplification.
Disadvantages
Cell Smear: Adhere non-adherent cells on coverslip with chemical bond
Cross-reactivity might occur with the secondary antibody, resulting in nonspecific signal.
An extra incubation step is required in the procedure.