Pregnancy testing kits use monoclonal antibodies to detect the hormone hCG in urine. Explain how monoclonal antibodies make this test specific to hCG and describe what happens in the test when hCG is present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Pregnancy testing kits contain monoclonal antibodies that are specific to the hormone human chorionic gonadotropin (hCG), which is only present in the urine of pregnant women.
Model answer (4 marks)
Monoclonal antibodies are produced from a single clone of B‑cells, giving them a single, highly specific binding site that matches the shape of the hCG hormone.
Because the binding site is unique, the antibodies will only attach to hCG and will not bind to other urinary substances.
When a urine sample containing hCG is applied to the test strip, the hCG molecules bind to the immobilised monoclonal antibodies.
This antibody–antigen complex is then detected by a secondary conjugate that produces a visible colour line, giving a positive result.
Because the binding site is unique, the antibodies will only attach to hCG and will not bind to other urinary substances.
When a urine sample containing hCG is applied to the test strip, the hCG molecules bind to the immobilised monoclonal antibodies.
This antibody–antigen complex is then detected by a secondary conjugate that produces a visible colour line, giving a positive result.
Examiner tips
- Use the word ‘specific’ and mention the single binding site of monoclonal antibodies
- Explain that only hCG will bind, not other antigens
- Describe the binding step and the visible colour change
- Keep each point short and to the point
Common mistakes
- Confusing monoclonal with polyclonal antibodies
- Failing to state that the antibodies bind only to hCG
- Not mentioning the visible colour change or line formation
Mark scheme (4 marks)
- Monoclonal antibodies have a specific shape / binding site complementary to hCG (antigen)
- So they will only bind to / attach to hCG and not other substances / antigens
- When hCG is present, it binds to the monoclonal antibodies on/in the test strip
- This causes a visible colour change / line to appear, indicating a positive result
Key terms in this question
Related
- Revision notes: Monoclonal antibodies (Bio only) →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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