ELISA test plate. Multi-well sample tray during an enzyme-linked immunosorbent assay (ELISA) blood test. This test checks for the presence of proteins such as antibodies and antigens, as well as allergens like pollen. Antibodies are produced by the immune system in response to antigens from disease-causing organisms. In a viral ELISA test, the patient's blood plasma is washed over a surface covered in the antigens of the virus being tested for. Any antibodies that are present bind to these antigens. Next, peroxidase is added which binds to the antibodies. Finally, a chemical is added that changes colour when it comes in contact with peroxidase.