A patient is diagnosed with HIV. Over several years, the number of white blood cells in the patient's body decreases significantly. Explain why this patient becomes increasingly unable to fight off other infections, and describe what this condition is called when the immune system can no longer function effectively.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
HIV is a virus that targets and destroys specific white blood cells involved in the immune response. A patient infected with HIV may initially show few symptoms, but over time the immune system becomes severely compromised.
Model answer (5 marks)
1. White blood cells, mainly lymphocytes, recognise and destroy invading pathogens.
2. HIV infects and destroys CD4+ T‑cells, a type of lymphocyte.
3. The loss of CD4+ cells reduces the total lymphocyte count.
4. With fewer lymphocytes, fewer antibodies are produced and the immune response is weakened.
5. When the immune system can no longer function effectively the condition is called AIDS (Acquired Immune Deficiency Syndrome).
2. HIV infects and destroys CD4+ T‑cells, a type of lymphocyte.
3. The loss of CD4+ cells reduces the total lymphocyte count.
4. With fewer lymphocytes, fewer antibodies are produced and the immune response is weakened.
5. When the immune system can no longer function effectively the condition is called AIDS (Acquired Immune Deficiency Syndrome).
Examiner tips
- Use the exact terms ‘lymphocytes’, ‘antibodies’, ‘CD4+ T‑cells’ and ‘AIDS’.
- Show the causal chain: HIV → loss of lymphocytes → fewer antibodies → ineffective immunity → AIDS.
Common mistakes
- Confusing HIV with the symptoms of AIDS; they are different stages.
- Using vague phrases like ‘immune system is weak’ instead of ‘immune deficiency’ or ‘AIDS’.
Mark scheme (5 marks)
- White blood cells (lymphocytes) produce antibodies to fight infection
- HIV destroys/reduces the number of white blood cells (lymphocytes)
- Fewer white blood cells means fewer antibodies can be produced
- The body cannot mount an effective immune response / cannot recognise and destroy pathogens
- This condition is called AIDS (Acquired Immune Deficiency Syndrome) / the immune system is described as deficient or compromised
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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