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The ELISA method takes advantage of the natural property of antigens and antibodies to bond together. A plastic dish with many wells in it is coated with an antibody for a particular antigen. Then a different sample is added to each well - for example, blood samples from different people. Several wells will contain positive and negative control samples. If the antigens in the blood match the antibody in the well, they will bind. Those that do not bind will be washed off. A second antibody is then added to the wells, which will only attach to the antigens. This second antibody is attached to an enzyme ("enzyme-linked") that will produce a colour when a solution is added to it. The entire dish can then be read by a scanner that looks for the presence of the enzyme's colour. If a colour is present, it means that sample contained the antigen of interest. If there is no colour, there was no antigen to bind to the first antibody. The control samples are used to make sure the procedure was successful - the positive control should be coloured and the negative control should not.
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