Monoclonal antibodies and ELISAAQA A-Level Biology: Revision notes

Monoclonal antibodies are identical antibodies, all produced by a single clone of plasma cells. Because they are identical, every antibody has the same tertiary structure at its variable region (binding site), so each type binds to one specific antigen (one specific epitope). A mixture of different antibodies from many clones is described as polyclonal.

Details of how monoclonal antibodies are produced commercially are not required; you need to know how they are used and the issues they raise.

A therapeutic drug can be attached to a monoclonal antibody. The antibody is chosen to bind to an antigen found only (or mainly) on the target cells, for example a receptor on the surface of a cancer cell.

• The antibody–drug complex travels in the blood and the antibody binds to the target cells
• The drug is delivered to these cells, so it acts where it is needed
• Healthy cells without the antigen are not affected, so there are fewer side effects than with conventional chemotherapy, which affects all dividing cells

Monoclonal antibodies can also carry a dye or radioactive label to show where a target cell type is in the body.

Monoclonal antibodies are used in diagnosis because they bind to one specific antigen.

• Pregnancy tests: mobile antibodies to hCG carry a coloured bead; hCG in urine binds to them, the complex is trapped by immobilised antibodies at the test line and a coloured line appears. Unbound mobile antibodies bind at the control line, showing the test has worked.
• Antibodies can detect antigens from pathogens or markers released by diseased cells, for example in blood samples

The specificity of monoclonal antibodies means fewer false positives, because they do not bind to similar molecules.

ELISA (enzyme-linked immunosorbent assay) uses antibodies to detect a specific antigen (or antibody) in a sample, and the enzyme produces a colour change.

1. The sample is added to a well so that any antigen sticks to the surface; the well is washed
2. A first antibody specific to the antigen is added; it binds to the antigen; the well is washed to remove unbound antibody
3. A second antibody, with an enzyme attached, binds to the first antibody; the well is washed
4. A colourless substrate is added; the enzyme converts it to a coloured product

The more intense the colour, the more antigen was present. If the antigen is absent, no first antibody is held, so no enzyme remains and there is no colour change.

Use of monoclonal antibodies raises ethical issues that you must be able to discuss.

• Animals: mice are used to produce the antibodies, which causes suffering
• Clinical trials: volunteers face risks of serious side effects and need informed consent; some patients may be given a placebo or the older treatment
• Cost and access: the treatments are expensive, so who gets them is an issue
• Benefits: targeted treatment with fewer side effects and quicker, more specific diagnosis

Give a balanced argument and a justified conclusion.

For trials of antibody treatments, comment on:

• Sample size and whether patients were randomly allocated to groups
• Whether a control (placebo or standard treatment) was used
• Whether the difference is statistically significant, not just a bigger number
• Whether the sample (age, sex, health) is representative, and whether the trial is long enough to show long-term effects
• Who paid for the study and whether other scientists have repeated it

A correlation does not prove that the treatment caused the change unless other variables were controlled.