Conjugated antibodies are primary antibodies that are attached to a substrate such as a toxin, enzyme or inorganic compound. They are also known as loaded, tagged or labeled antibodies. Proteins conjugated with fluorescent dye are used extensively in several modern immunoassay techniques including Western blot, ELISA, flow cytometry, immunohistochemistry, and immunofluorescence among others.

Unconjugated antibodies are primary antibodies that are not attached to any substrate.