Fluorescent antibody-stained image of Neisseria gonorrhoeae. In a direct fluorescent antibody test, antibodies that have been chemically linked to a fluorescent dye, i.e. the marker, are added to a specimen suspected of containing a pathogen, in this case N. gonorrhoeae bacteria. If the pathogen is present, the antibodies link to the antigen to which they are sensitive - the N. gonorrhoeae bacterial cell wall, here - and under fluorescent microscopy, emit a green signal.