A person with HIV is said to have a weakened immune system. Explain why a person with HIV is more likely to develop serious infections than a healthy person, and describe the role of both T-lymphocytes and B-lymphocytes in a normal immune response to a pathogen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
HIV (Human Immunodeficiency Virus) is an infectious virus spread by direct contact with infected body fluids. It destroys white blood cells, weakening the immune system and may lead to AIDS (acquired immunodeficiency syndrome).
Model answer (5 marks)
HIV infects and destroys CD4+ T‑lymphocytes, the main white blood cells that recognise and coordinate the immune response. With fewer T‑cells, the body cannot mount an effective defence against invading microbes, so infections that are normally cleared become serious.
In a normal response, T‑lymphocytes recognise specific antigens on a pathogen’s surface. They then activate B‑lymphocytes, signalling them to differentiate into plasma cells. The plasma cells produce antibodies that bind the same antigen, neutralising the pathogen and marking it for destruction.
Because HIV reduces the number of T‑cells, B‑cells receive little or no activation signal. Consequently fewer antibodies are produced, the pathogen is not neutralised, and the infection can become severe.
In a normal response, T‑lymphocytes recognise specific antigens on a pathogen’s surface. They then activate B‑lymphocytes, signalling them to differentiate into plasma cells. The plasma cells produce antibodies that bind the same antigen, neutralising the pathogen and marking it for destruction.
Because HIV reduces the number of T‑cells, B‑cells receive little or no activation signal. Consequently fewer antibodies are produced, the pathogen is not neutralised, and the infection can become severe.
Examiner tips
- Use the word ‘destroy’ for HIV’s effect on T‑cells; link this to reduced antibody production.
- Show the sequence: T‑cell recognition → B‑cell activation → antibody production.
- Mention CD4+ T‑cells specifically to demonstrate knowledge of the cell type involved.
Common mistakes
- Confusing T‑cells with B‑cells as the primary cells destroyed by HIV.
- Failing to explain the activation step from T‑ to B‑cells.
- Omitting the role of antibodies in neutralising pathogens.
Mark scheme (5 marks)
- HIV destroys white blood cells, so the immune system is weakened / fewer white blood cells available to respond to pathogens
- T-lymphocytes detect / respond to antigens (unique chemicals) on the surface of a pathogen
- T-lymphocytes stimulate / activate B-lymphocytes to respond to the antigen
- B-lymphocytes produce antibodies specific to the antigen on the pathogen
- Because HIV destroys white blood cells (including T-lymphocytes), B-lymphocytes cannot be activated / fewer antibodies are produced, so the immune response is ineffective and serious infections develop
Key terms in this question
HIV · T-lymphocyte · B-lymphocyte
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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