And so following the washing process, if we add the substrate because no enzyme is present in that mixture, the color change would not actually take place. So the color change signifies the presence of that antibody that we are actually searching for in the first place. Now, the darker that color change is, the higher the concentration of that antibody inside our mixture. So this method not only allows us to actually determine the presence of the antibody, it also allows us to quantify to determine what the concentration is of that particular antibody in our mixture. So the entire point of this indirect Eliza method is to basically test for that presence of our antibody inside that sample, for example, inside that blood sample. Now, if the individual did not have HIV, they would not have these blue antibodies.