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ELISA stands for enzyme-linked immunosorbent assay. It is a technique often used to test for antibodies and antigens in human blood samples. ELISAs are performed in plastic, multi-well plates. There are many types of ELISAs, but all share common steps. First, the antigen or antibody is immobilized to the plate. If detecting antigens, capture antibodies are linked to the bottom of each well that bind to the target antigen and immobilize it. If detecting antibodies, antigens are linked to the bottom of each well that binds the target antibody to immobilize it. The plate is then washed to remove any unbound antigen or antibody. Second, a detection antibody is added to each well. The detection antibody binds to the immobilized antigen or antibody. It is also linked to an enzyme that can catalyze a reaction that often produces a color change. The plate is then washed to remove any detection antibody. Third, the substrate for the enzyme is added. If there is target antigen or antibody present in the sample, a color change in the well will occur. The higher the concentration of antigen or antibody in a sample, the stronger the color change will be.