FRET-based immunosensors are analytical tools that utilize FRET to detect and quantify specific antigens or antibodies in a sample. These sensors typically involve the use of fluorescently labeled antibodies or antigens, where one fluorophore acts as a donor and the other as an acceptor. When the target antigen binds to its corresponding antibody it brings the donor and acceptor fluorophores into close proximity.  Energy transfer then occurs, leading to a measurable change in fluorescence signal.