ELISA (AQA A Level Biology)

• Each clone of plasma cells produces only one type of antibody
  • These are known as monoclonal antibodies

• Monoclonal antibodies can be used in medical diagnoses
  • They can be used in test kits to diagnose diseases or conditions
  • These type of tests are very quick and reliable
  • ELISA tests are an example of these type of tests

ELISA tests

• ELISA stands for enzyme-linked immunosorbent assay
• ELISA tests can be used to see if a patient has any antibodies to a certain antigen (or any antigens to a certain antibody)
  • For example, they can be used to test for infections by pathogens or for allergies

• In an ELISA test:
  • An enzyme is attached to antibodies
  • When this enzyme reacts with a certain substrate, a coloured product is formed, causing the solution in the reaction vessel to change colour
  • If a colour change occurs, this shows that the antigen or antibody of interest is present in the sample being tested (e.g. blood plasma)

• There are different types of ELISA test:
  • Direct ELISA tests use a single antibody that is complementary to the antigen being tested for
  • Indirect ELISA tests use two different antibodies (known as primary and secondary antibodies)

Example of an indirect ELISA test: testing for antibodies

• An indirect ELISA test can be used to test whether a patient has antibodies to HIV:
  • First, HIV antigens are bound to the bottom of the reaction vessel
  • A blood plasma sample is then taken from the patient and added to the reaction vessel
  • Any HIV-specific antibodies (i.e. the antibodies produced against HIV) that are present in the blood plasma now bind to the HIV antigens (stuck to the bottom of the reaction vessel). These HIV-specific antibodies are known as the primary antibodies
  • Any other antibodies that are present in the blood plasma are unbound and are washed out
  • A second type of antibody with an enzyme attached to it is added to the reaction vessel. These are known as the secondary antibodies
  • These secondary antibodies bind to the primary antibodies. The reaction vessel is washed out again to remove any unbound secondary antibodies. This is a very important step in avoiding false-positive test results. If they are not washed out, unbound secondary antibodies would give a positive result, even if there were no primary (HIV-specific) antibodies present to start with
  • Finally, a solution is added that contains a substrate that reacts with the enzyme attached to the secondary antibodies. If there are any secondary antibodies present, a coloured product is formed, causing the solution in the reaction vessel to change colour. This indicates that the patient has HIV-specific antibodies in their blood (and therefore they are infected with HIV)

An indirect ELISA test can be used to test whether a patient has antibodies to HIV

Example of an indirect ELISA test: testing for antigens

• Another type of indirect ELISA test can be used to test whether a patient has specific antigens in their blood plasma
• For example, this can be used to test for prostate cancer:
  • Prostate cancer is a cancer of the prostate gland (so only occurs in men)
  • The blood plasma of a patient can be tested for the presence of prostate-specific antigens (PSAs)
  • If the PSA concentration of the blood plasma is abnormally high, this suggests that the patient has prostate cancer and further diagnostic tests will be carried out

• This indirect ELISA test is very similar to the one described for HIV above
• The main difference is that antibodies to PSA are bound to the bottom of the reaction vessel (instead of antigens, as in the HIV example)

An indirect ELISA test can be used to test whether a patient has prostate-specific antigens