 Alternatively, if the analyte itself is an antibody, its target antigen can be used as the binding reagent.Before the development of the ELISA, the only option for conducting an immunoassay was radioimmunoassay, a technique using radioactively labeled antigens or antibodies.In radioimmunoassay, the radioactivity provides the signal, which indicates whether a specific antigen or antibody is present in the sample.