ELISA is an abbreviation of enzyme linked immunosorbent assay and utilize the bond between an antibody and its specific antigen for it to work. There are 4 main types of ELISA:

1. Direct ELISA
2. Indirect ELISA
3. Sandwich ELISA
4. Competitive ELISA

Today we will look closer at sandwich ELISA which is carried out in the following manner:

1. First, capture antibodies specific for the target analyte are bound to the floor of a micro well plate.
2. Second, the sample is added and any antigens, present in the sample, that are specific to the capture antibodies bind to them. Then the well is washed to remove all unbound sample components.
3. Third, enzyme-conjugated antibodies which are also specific to the target analyte are added which bind to the antigens on the target analyte.
4. Finally, substrate for the enzyme which is linked to the second antibody is added and the enzyme converts this substrate into an observable signal.

This means that if the target antigen is present, it gets bound by the capture antibody. Then the enzyme-conjugated antibody can bind to the target analyte yet again and in its turn produce a signal once the substrate is added, producing a positive test result. If the target antigen is not present however, this prevents the enzyme-conjugated antibody from binding to them which in turn produces a negative test result.

In other words, if a color change occurs, it is interpreted as a positive test result and if no color change occurs it is interpreted as a negative test result.