Before you read this page
This page builds on the Cell Recognition & the Immune System page. Make sure you’re comfortable with helper T cells and the cellular response first.
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
- Structure of the human immunodeficiency virus (HIV) and its replication in helper T cells.
- How HIV causes the symptoms of AIDS.
- Why antibiotics are ineffective against viruses.
Human Immunodeficiency Virus (HIV) infects and destroys helper T cells, a key part of the body’s specific immune response.
Over time, helper T cell numbers drop. When they get low enough, the immune system can no longer fight pathogens effectively. This stage is called AIDS (Acquired Immunodeficiency Syndrome). The body becomes vulnerable to opportunistic infections (e.g. TB, pneumonia) and certain cancers, which cause the symptoms of AIDS.
Without helper T cells, which immune cells can no longer be activated, and what's the consequence?
Without helper T cells, which immune cells can no longer be activated, and what's the consequence?
Without helper T cells, the body cannot activate:
- Cytotoxic T cells (so virus-infected and cancerous cells aren’t destroyed)
- B cells (so antibodies aren’t produced in sufficient quantities)
- Phagocytes (so the non-specific response is also weakened)
- This means the immune system can no longer fight off infections
Structure of HIV
HIV is a retrovirus, meaning its genetic material is RNA not DNA. HIV uses reverse transcriptase to make DNA from RNA (see process later).
- RNA: the viral genome
- Reverse transcriptase: an enzyme that converts viral RNA into DNA inside the host cell
- Capsid: protein coat enclosing the genetic material
- Attachment proteins: on the surface, bind to receptors (CD4) on helper T cells
- Lipid envelope: outer layer (taken from the host cell membrane during budding)
Replication of HIV in Helper T Cells
HIV can only replicate inside a host cell. It has no ribosomes of its own so it can’t make proteins.
- The attachment protein on HIV binds to a CD4 receptor on the helper T cell
- The lipid envelope fuses with the host cell membrane, releasing the RNA and reverse transcriptase into the cell
- Reverse transcriptase converts the viral RNA into DNA (as the name suggests it’s the reverse of transcription)
- This viral DNA is incorporated into the host cell’s DNA (in the nucleus)
- The viral DNA is transcribed into mRNA, then host cell ribosomes translate the mRNA into viral proteins
- New HIV particles are assembled and bud off from the host cell (taking some membrane as the new lipid envelope), often killing the helper T cell
Antibiotics & Viruses
Why are antibiotics ineffective against viruses?
Antibiotics target structures or processes that are specific to bacteria (e.g. cell walls / murein, bacterial ribosomes).
Viruses have none of these structures:
- Have no cell wall
- Have no ribosomes of their own (they use the host cell’s)
This is why HIV (and other viral infections like flu, colds) can’t be treated with antibiotics. Only antiviral drugs (e.g. reverse transcriptase inhibitors) work, by targeting virus-specific processes.
HIV is examined as part of cell recognition and the immune system. See how it's tested on that page →
Tips from examiner reports
What students commonly get wrong
- HIV's genetic material is RNAWatch out: HIV's genetic material is RNA, not DNA. 2019 P1 Q5.1 · 2024 P3 Q1.3
- HIV replication: be specificWhen describing how HIV replicates inside helper T cells, give the specific steps, e.g. reverse transcriptase makes DNA from the viral RNA, the DNA is inserted into the host DNA, then transcribed into mRNA, which is translated into viral proteins. "New viruses are made using the cell's own machinery" is too vague. 2021 P3 Q2.1
- Why there's no HIV vaccineHIV mutates quickly, so its antigens keep changing (antigenic variability). Memory cells made after a vaccine wouldn't recognise the new antigens. Watch out: "HIV has no cell wall" explains why antibiotics don't work, not vaccines. 2024 P3 Q1.2
Exam Question Practice
A few exam-style questions on this topic to try. Have a go at each one before opening the answer.
Describe the structure of the human immunodeficiency virus (HIV).
(4 marks)
Hint
Work from the inside out: what is the genetic material, which enzyme is packaged with it, and what layers surround them?
Mark Scheme
Max 4 marks
- RNA (as genetic material) (1 mark)
- Reverse transcriptase (1 mark)
- (Protein) capsomeres/capsid (1 mark)
- (Phospho)lipid (viral) envelope
OR Envelope made of membrane (1 mark) - Attachment proteins (1 mark)
Also accepted / rejected
Tips
- Don’t confuse viruses with bacteria: HIV has no cell membrane, cell wall or capsule
- Watch for contradictions later in your answer: HIV’s genetic material is RNA, not DNA, and attachment proteins aren’t antibodies
- A labelled diagram can help, but it isn’t needed
Describe how the human immunodeficiency virus (HIV) is replicated once inside helper T cells (TH cells).
(4 marks)
Hint
HIV’s genetic material is RNA. What enzyme turns it into DNA, where does that DNA go, and how are new viral proteins made from it?
Mark Scheme
- RNA converted into DNA using reverse transcriptase (1 mark)
- DNA incorporated/inserted into the (helper T cell’s) DNA/chromosome/genome/nucleus (1 mark)
- DNA transcribed into (HIV m)RNA (1 mark)
- (HIV mRNA) translated into (new) HIV/viral proteins (for assembly into viral particles) (1 mark)
Also accepted / rejected
1. Reject “messenger” or “m” before RNA.
3. Accept descriptions of transcription.
4. Accept descriptions of translation. Accept a named viral protein, e.g. capsid. Reject “viral cells”.
Tips
- Give each step with its enzyme or process: reverse transcriptase makes DNA from the viral RNA; the DNA is inserted into the TH cell’s DNA; it’s transcribed into mRNA and translated into viral proteins
- “New viruses are made using the cell’s machinery” is too vague
- It’s viral RNA, not mRNA, that is reverse transcribed
Most people infected with HIV go on to develop AIDS if they are not treated. A small group of people, called HIV controllers, stay healthy without treatment.
Scientists compared blood samples from HIV controllers with blood samples from patients who had AIDS. In each sample they measured the viral load (the number of HIV particles) and the number of CD4 cells, a type of helper T cell.
Table 1 shows the median and range for each group.
Table 1
| Group | Median viral load / particles per cm³ of blood (range) | Median number of CD4 cells per mm³ of blood (range) |
|---|---|---|
| HIV controllers | 50 (0 to 2000) | 750 (400 to 1300) |
| Patients with AIDS | 120 000 (8000 to 750 000) | 150 (10 to 380) |
Use the data in Table 1 and your knowledge of the immune response to suggest why HIV controllers do not develop symptoms of AIDS.
(3 marks)
Hint
Compare the two groups in Table 1. What do helper T cells normally activate, and why does losing them lead to the symptoms of AIDS?
Mark Scheme
Max 3 marks
- (All) have more helper T/CD4 cells (1 mark)
- Lower viral load to infect/destroy helper T/CD4 cells (1 mark)
- (So more/continued) activation of B cells/cytotoxic T cells/phagocytes (1 mark)
- (With B cells, more/continued) production of plasma cells/antibodies
OR (With cytotoxic T cells, more/continued) ability to kill virus-infected cells (1 mark) - (More able to) destroy other microbes/pathogens
OR (More able to) destroy mutated/cancer cells (1 mark)
Also accepted / rejected
1. Accept a higher proportion of helper T/CD4 cells to virus particles.
1 and 2. The statement must be comparative.
2. For infect accept “HIV does not reproduce in”.
3. Accept stimulation for activation.
4. Ignore reference to B cells acting as phagocytes/antigen-presenting cells.
Tips
- Use both columns of Table 1 and compare the two groups
- Link the immune response to AIDS symptoms: with more helper T cells, the body can still destroy other pathogens and cancer cells; this final link is very often missed
A doctor does not prescribe antibiotics to a patient with flu.
Give one reason why antibiotics don’t work against viruses.
(1 marks)
Hint
Antibiotics target specific bacterial structures. What structures do bacteria have that viruses lack?
Mark Scheme
Viruses don’t have the structures or enzymes that antibiotics target, e.g. no cell wall (murein), or no metabolism of their own (1 mark)
Also accepted / rejected
Also accepted
- “No ribosomes” of their own
- A named process, e.g. they don’t make their own proteins
Not accepted
- “Viruses hide inside cells”
Tips
- Name what antibiotics target that viruses lack, e.g. a cell wall (murein) or bacterial enzymes
- “Viruses hide inside cells” is a GCSE-level answer that gains no mark
- Don’t confuse antibiotics with antibodies
Table 1 shows how two antibiotics work.
Table 1
| Antibiotic | How it works |
|---|---|
| Nalidixic acid | Inhibits an enzyme that bacteria need to replicate their DNA |
| Vancomycin | Stops cross-links forming between murein (peptidoglycan) molecules in bacterial cell walls |
Using the information in Table 1, explain why each antibiotic is not effective against HIV.
(2 marks)
Hint
What type of genetic material does HIV have? Remember viruses don’t have the same structures as bacteria.
Mark Scheme
Nalidixic acid
- HIV has RNA, not DNA (1 mark)
Vancomycin
- HIV has no cell wall (no murein) (1 mark)
Also accepted / rejected
Not accepted
- “Single-stranded DNA” (point 1)
- Viruses having a cell membrane, or a cell wall made of chitin (point 2)
Tips
- HIV’s genetic material is RNA, so don’t say it has single-stranded DNA
- Say HIV has no cell wall (no murein). Don’t describe viral structures wrongly, such as a cell wall made of chitin
- Viruses aren’t cells, so they don’t have a cell membrane. HIV’s outer layer is a lipid envelope
Accept a labelled diagram.
1. Reject nucleus/DNA/plasmids.
3. Reject capsule.
4. Reject if HIV is said to have a cell membrane or a cell wall.
5. Accept gp41 and/or gp120. Accept glycoprotein. Accept a description of an attachment protein. Ignore “receptor protein”.
Ignore cytoplasm.