A laboratory technique to identify specific antibodies or antigens. Antibody identification is usually performed on blood (serum). The exact technique may vary depending on the specific antibody that is being investigated and between different laboratories. In general, cells, tissue or some other substance is placed onto microscope slides. A small amount of sample containing antibodies (typically serum -- a liquid portion of blood) is placed over the cells or tissue allowing the antibodies that are specific for particular tissue or cellular antigens to bind. The serum is washed away, and a second antibody that binds to human antibodies (often made in another animal species such as rabbits or goats) is applied to the slide. This second antibody has a fluorescent dye chemically linked to it. If the person's serum has antibodies that bind to the tissue or cells, a bright fluorescence can be seen by use of a special microscope.