The labeled antigen competes for primary antibody binding sites with the sample antigen (unlabeled).

The more antigen in the sample, the more labeled antigen is retained in the well and the stronger the signal.

This FITC labeled antigen emits light at a wavelength of 530 nm.

Before the development of the ELISA, the only option for conducting an immunoassay was radioimmunoassay, a technique using radioactively labeled antigens or antibodies.

The CD63 marker is an FITC labeled antigen which can bind to an CD63 protein and is used to sort the cells via FACS(Fluorescence activated cell sorting/sorter).