Immunolabelling is a technique used to detect proteins either in a cell or in an experiment such as Western Blotting. An antibody will recognise when the desired protein is added, in the case of a cell a method must be devised to get the antibody inside. The antibody (primary antibody) specifically binds to the antigen. In a Western Blotting experiment this takes about an hour and a half on a moving plate. Then you add a secondary antibody which recognises the primary antibody as an antigen. The secondary antibody is conjugated with a fluorescent marker, so the location of your protein can be detected using this technique.