Known HIV antigens are adsorbed
to test well. The patient's
serum is added. If the serum contains antibodies against the known HIV antigens,
they will bind to those antigens. All other antibodies are then washed from
the well. 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 well. The substrate for the enzyme attached to the anti-HGG
is added to the well. The enzyme substrate reaction produces a visible color
change which can be measured with a spectrophotometer. This shows that the patient's
serum must have contained antibodies against the known HIV antigens. 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.