The elisa are of two types . they are DAS ELISA –double antibody sandwich ELISA, and DAC ELISA – direct antigen coating ELISA .the first one is used to detect the antigen and second is used to find antibodies

The sample of blood suspected to contain antigen is take on the surface of an ELISA plate. After this the antibody specific to the antigen is added and allowed to react them. After complition of reaction the un reacted antibodies are washed out . now the plate contains only the antibody bound antigen . this antigen antibody complex is again allowed to react with anti immuno globulin. Then the complex is linked to an enzyme known as peroxidase . the un reacted part of the complex is washed away. , add the substrate of the enzyme along with the reagents. The enzyme reacts with the substrate and give a colored product .the intensity of color depends on the amount of antigen in the blood. ELSA is the primary test for finding hiv virus, after this other confirmation test is to do.