Monoclonal Antibodies in Medicine
How monoclonal antibodies deliver a drug to cancer cells and show a substance is present in a sample, using specific binding to one antigen. Includes the 4-mark exam question, a worked model answer and the examiner trap on saying monoclonal antibodies kill cancer cells.
Chapters (7)
Exam question [4 marks]
Monoclonal antibodies are used to treat some cancers and to diagnose disease. Explain how a monoclonal antibody can be used (a) to deliver a drug to cancer cells [3 marks] and (b) to show that a substance is present in a sample [1 mark].
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Antibody has a binding site complementary to an antigen found on (only) the cancer cells, so it binds specifically to them | "specific to cancer cell antigen", "complementary shape", "binds to receptor/antigen on cancer cells" | "antibody attacks / recognises cancer" with no antigen or binding; "antibody has the same shape as the cancer cell" |
| 2 | Drug is attached to the antibody, so the drug is delivered to / held against the cancer cells | "antibody carries the drug", "radioactive or toxic substance attached to the antibody", "drug targeted at cancer cells" | "antibody kills the cancer cells"; "antibody is the drug" |
| 3 | Healthy cells are far less affected / fewer side effects, because the antibody does not bind to them | "drug concentrated at target", "does not bind to normal cells" | "no side effects at all"; "healthy cells are not killed" with no reason |
| 4 | (Diagnosis) a labelled antibody binds to the specific antigen in the sample, so the label (e.g. colour change) shows the antigen is present | "dye / enzyme / marker attached", "colour shows the substance is there", "second labelled antibody binds the antigen and gives a signal" | "antibody tests for disease" with no binding or label; "antibody changes colour" with no binding |
Mark 3 needs an explicit statement about healthy cells or side effects. Marks 1 to 3 are for targeted therapy and mark 4 is for diagnosis. Every mark rests on specificity: a binding site complementary to one antigen. The antibody delivers the drug and does not kill cells itself. Details of how monoclonal antibodies are produced are not required by the specification and score nothing here.
Model answer (4/4)
The monoclonal antibody has a binding site complementary to an antigen found only on the cancer cells, so it binds specifically to those cells. A therapeutic drug is attached to the antibody, so the drug is delivered to the cancer cells. Healthy cells are far less affected because the antibody does not bind to them. For diagnosis, an antibody with a label attached binds to the specific antigen in the sample, so the label shows that the antigen is present.
Examiner trap
- Saying monoclonal antibodies 'kill cancer cells' directly
- Not saying the antibody binds to a specific antigen on the target cells
- Writing about production methods that are not required
Key terms
- monoclonal antibody
- specific
- complementary
- target cell
- therapeutic drug
- diagnosis
- antigen
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:30Antibodies are specific
A monoclonal antibody is a large number of identical antibodies, all with the same binding site. That binding site has a shape that is complementary to one particular antigen. So the antibody binds to that antigen, and to nothing else. It is specific.
Everything in this lesson comes from that one idea. Pick the right antigen, and you can send the antibody exactly where you want.
0:56Targeting cancer cells
Now, cancer cells often have antigens on their cell surface membrane that healthy body cells do not have. So we make a monoclonal antibody with a binding site complementary to that cancer antigen.
1:10Attaching a therapeutic drug
Next, we attach a therapeutic drug to the antibody. The antibody is injected, and travels in the blood. It passes by healthy cells, because its binding site is not complementary to any antigen on them, so it does not bind. But when it meets a cancer cell, it binds to the antigen, and holds the drug right against the target cells.
So the antibody does not kill the cancer. It is the drug that does that. The antibody is the delivery van that knows the address. The drug is concentrated where it is needed, and healthy cells are far less damaged, so there are fewer side effects.
1:48Using antibodies in diagnosis
Now diagnosis. Here the antibody carries a label instead of a drug. It might be a dye, or an enzyme that causes a colour change. Add the sample. If the antigen is present, the antibody binds to it. The label is held in place, and we can see it, or measure it. No antigen, no binding, no signal.
A pregnancy test works this way. The antibody binds to a hormone in the urine, and a coloured line appears. Other tests find antigens from pathogens, or from cancer cells.
2:59Examiner trap and recap
The trap. Do not say the antibody kills the cancer cells. Say it binds to a specific antigen on those cells, and delivers the drug. You do not need to describe how they are made.
Recap. Specific binding site, drug or label attached, and you can target or detect one antigen.
Next, we look at the ELISA test, which uses antibodies to detect an antigen very precisely.
Watch first
This lesson builds on:
- B Cells, Clonal Selection and Antibodies · AQA 7402 · 3.2.4
Keep going
More from 3.2 Cells
- Antigens and Cell Recognition · AQA 7402 · 3.2.4
- Phagocytosis: How Phagocytes Work · AQA 7402 · 3.2.4
- T Lymphocytes: The Cellular Immune Response · AQA 7402 · 3.2.4
- B Cells, Clonal Selection and Antibodies · AQA 7402 · 3.2.4
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-048.