Explain how monoclonal antibodies can be used in a pregnancy test to detect whether a woman is pregnant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Pregnancy tests detect a hormone called human chorionic gonadotropin (hCG), which is only present in the urine of pregnant women. Monoclonal antibodies specific to hCG are used in these tests.
Model answer (5 marks)
Monoclonal antibodies are produced so that they are highly specific for the pregnancy hormone hCG.
In the test strip, hCG present in the urine binds to these antibodies.
The antibodies are linked to a coloured marker or dye.
When hCG is present, the antibody–hCG complex migrates to a test line where the dye gives a visible colour change, indicating a positive result.
If hCG is absent, no complex forms, no dye accumulates at the test line and no colour change is seen, giving a negative result.
In the test strip, hCG present in the urine binds to these antibodies.
The antibodies are linked to a coloured marker or dye.
When hCG is present, the antibody–hCG complex migrates to a test line where the dye gives a visible colour change, indicating a positive result.
If hCG is absent, no complex forms, no dye accumulates at the test line and no colour change is seen, giving a negative result.
Examiner tips
- Use the exact terms ‘antigen’, ‘antibody’, ‘coloured marker’, ‘positive/negative result’.
- Show the sequence: antibody production → binding → dye linkage → colour change at test line.
Common mistakes
- Confusing monoclonal with polyclonal antibodies.
- Forgetting to mention the dye or colour change as the detection method.
- Claiming the test works by detecting hCG in blood instead of urine.
Mark scheme (5 marks)
- Monoclonal antibodies are produced to be specific / complementary to hCG (the antigen)
- hCG in the urine binds to / attaches to the monoclonal antibodies
- The antibodies are attached to / linked to a coloured marker / dye / indicator
- If hCG is present, the antibody–antigen complex moves to / collects at a specific region and produces a colour change / visible result / positive result
- If hCG is absent (woman is not pregnant) no colour change / no visible result / negative result is produced
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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