The principle of the double antibody sandwich ELISA is represented in Figure 1. In this assay the IgG present in the sample reacts with the anti-IgG antibody which has been adsorbed to the surface of polystyrene microtiter wells. After the removal of unbound sample proteins by washing, anti-IgG antibody conjugated with horseradish peroxidase (HRP) is added. This HRP-conjugated antibody forms a complex with the previously bound IgG. Following another washing step, the enzyme bound to the immunosorbent is assayed by the addition of a chromogenic substrate, 3,3