If using a procedure (such as the ABC method) that required a secondary antibody, the secondary is usually biotinylated or is conjugated with a fluorophore or chromogen. The secondary antibody must be an antibody against the primary antibody’s host.
Example: If the primary antibody is made in mouse the secondary or linking antibody should be horse anti-mouse.
The secondary antibody is also referred to as the linking antibody, where its purpose is to add a place for attachment of the avidin-biotin enzyme complex. In an immunofluorescence staining procedure, it is the last step if the secondary is conjugated with a fluorophore.