Different known HIV antigens are separated on a strip. The patient's serum is added. If the serum contains antibodies against any of the known HIV antigens, they will bind to those antigens on the strip. All other antibodies are then washed from the strip. Enzyme-linked anti-human gamma globulin (anti-HGG) is added to the well. (Anti-HGG is an antibody made by another animal against human IgG antibodies. An enzyme is then attached to the antibody.) The anti-HGG will with any human IgG antibodies bound to the adsorbed HIV antigens. All unbound anti-HGG is then washed from the strip. The substrate for the enzyme attached to the anti-HGG is added to the strip. The enzyme substrate reaction produces a visible color change. This shows that the patient's serum must have contained antibodies against the known HIV antigens on the strip where the reaction took place. If there were no antibodies present then there would be no enzyme-linked anti-HGG in the well and no color-producing enzyme-substrate reaction.