ELISA is used to detect viruses where the key reagent is:

Explanation

ELISA (enzyme linked immunosorbent assay) screening test is the initial test to diagnose AIDS. The test works by detecting antibodies/ substances or protein which are produced in the blood when the virus is present.Alkaline phosphatase and peroxidases are commonly used enzymes as key reagents to perform the ELISA test. These enzymes are used to provide antibody-­antigen complex in a specialized ELISA plate or tray. In ELISA test for detecting a particular antigen, its antibody is buffered and a drop of serum (supernatant of centrifuged blood) is poured over it. If the latter contains antigen, it will produceantigen­ antibody complex. A second enzyme labelled antibody is added. It forms an enzyme­antigen­ antibody complex if the antigen is present. The substrate is now added. It produces a stain if the antigen is present.