Immunofluorescence is a technique used in many biological research applications.
Samples are mixed with a solution containing fluorescently labelled antibodies against the target (antigen) of interest – in this case the SARS-CoV-2 nucleoprotein.
These form antigen–antibody complexes if the n-protein is present. These are captured in the test cassette and the immunocomplexes are detected by a fluorescence detector. The detector is very sensitive and can read signals invisible to the naked eye.