(a) : ELISA (enzyme linked immunosorbent assay) screening test is the initial test to diagnose AIDS. The test works by detecting anitibodies/ substances or protein which are produced in the blood when virus is present. Alkaline phosphatase and peroxidases are commonly used enzymes as key reagent to perform the ELISA test. These enzymes are used to provide antibody-antigen complex in a specialised 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) poured over it If the latter contains antigen, it will produce antigen antibody complex. A second enzyme labelled antibody is added. It forms enzyme-antigenantibody complex, if the antigen is present. Substrate is now added. It produces a stain if the antigen is present.