Vaccination and Immunity — AQA A-level Biology

SPEC 3.2.4 - CELLS — IMMUNITY

Monoclonal Antibodies, HIV & ELISA

HIV structure & replication · AIDS · Monoclonal antibody production · Uses of mAbs · ELISA · Ethical issues

HIV Structure

HIV (Human Immunodeficiency Virus) is a retrovirus — it carries its genetic information as RNA and uses reverse transcriptase to make DNA inside the host cell.

Key Structural Features

Lipid envelope derived from the host cell membrane when the virus buds out

Glycoprotein spikes (gp120) — bind specifically to CD4 receptors on helper T cells

Protein capsid — protects the genetic material

2 strands of RNA — the viral genome (not DNA)

Reverse transcriptase enzyme — converts RNA → DNA inside the host cell

HIV Replication in Helper T Cells

HIV specifically targets helper T cells because they carry CD4 surface receptors that complement HIV's gp120 glycoprotein spikes.

1

HIV's gp120 glycoprotein binds to the CD4 receptor on the helper T cell surface. The viral envelope fuses with the host cell membrane

2

HIV RNA and reverse transcriptase are released into the cytoplasm of the helper T cell

3

Reverse transcriptase catalyses the synthesis of viral DNA from the RNA template (reverse transcription: RNA → DNA)

4

Viral DNA is integrated into the host cell's chromosomes — at this stage the virus is called a provirus. It may remain dormant for years

5

When activated, the host cell's own transcription and translation machinery produces new viral RNA and proteins

6

New HIV particles bud from the host cell, taking a portion of the host cell membrane as their lipid envelope — the host T cell is eventually destroyed

AIDS & Why Antibiotics Fail Against HIV

From HIV to AIDS

HIV gradually destroys helper T cells over months to years

As helper T cell count falls, both cellular and humoral immune responses are impaired

AIDS (Acquired Immunodeficiency Syndrome) is diagnosed when the helper T cell count falls below 200 cells per microlitre (normal: 500–1500)

At this point the immune system cannot defend against opportunistic infections — pathogens that rarely cause disease in healthy people

Death is typically caused by these opportunistic infections, not HIV directly

Why Antibiotics Are Ineffective Against Viruses

Antibiotics target structures found in bacteria but absent in viruses

Bacterial targets: murein (peptidoglycan) cell wall, bacterial 70S ribosomes, bacterial cell surface

Viruses have none of these structures — they have no cell wall, no ribosomes of their own, and no metabolism outside a host cell

Antibiotics therefore have nothing to target in a viral infection

Examiner Alert ⚠️

A common exam error is to say "antibiotics kill viruses." They do not. The precise reason why is that antibiotics target bacterial-specific structures (cell wall, 70S ribosomes) that viruses do not possess. Always name the specific bacterial structure that is absent from viruses.

Monoclonal Antibody Production

Definition — Monoclonal Antibody

A monoclonal antibody is an antibody produced by a single clone of hybridoma cells; all are identical and specific for the same antigen.

The production method exploits two cell types with complementary properties:

1

A mouse (or other animal) is immunised with the target antigen — its B cells produce antibodies specific to that antigen

2

B cells (antibody-producing but mortal) are extracted from the mouse's spleen

3

B cells are fused with myeloma cells (immortal tumour cells that do not produce the desired antibody) to create cells with both properties

4

The fusion produces hybridoma cells — immortal AND able to produce the specific antibody

5

Hybridomas are screened and selected for production of the desired antibody, then cloned. They can be cultured indefinitely to produce large quantities of identical (monoclonal) antibodies

AQA Specification Note

AQA states: "Details of the commercial or scientific production of monoclonal antibodies are not required." You need to know the concept (B cell + myeloma cell → hybridoma) but not the detailed technical protocols.

Hybridoma Method — Diagram

Uses of Monoclonal Antibodies

Monoclonal antibodies are useful precisely because they are highly specific — each binds to only one antigen. This specificity enables targeted applications.

UseHow it WorksExample

Targeted drug deliveryA therapeutic drug (e.g. cytotoxic agent or radioactive label) is attached to the antibody. The antibody carries the drug specifically to cells expressing the target antigen, sparing healthy cellsCancer treatment — mAb targets antigen on cancer cell surface; cytotoxic drug delivered directly to tumour cells

Medical diagnosismAbs specific to a target molecule (hormone, pathogen antigen) detect its presence in a samplePregnancy tests detect hCG hormone; lateral flow tests detect pathogen antigens (e.g. COVID-19 LFT)

ELISAEnzyme-Linked ImmunoSorbent Assay — mAbs linked to enzymes detect and quantify specific antigens or antibodiesHIV antibody testing; detection of specific proteins in blood samples

Examiner Alert ⚠️

Students often say monoclonal antibodies "kill cancer cells." More precisely: the drug attached to the mAb kills the cancer cell — the antibody provides specificity of targeting. The antibody itself does not kill the cell; it delivers the cytotoxic agent selectively.

The ELISA Test

Definition — ELISA

ELISA (Enzyme-Linked ImmunoSorbent Assay) is a test that uses antibodies linked to enzymes to detect and quantify the presence of specific antigens or antibodies in a sample.

The key principle is that the enzyme-linked antibody produces a colour change when its substrate is added — only if the target antigen (or antibody) is present. The intensity of the colour indicates the quantity.

Basic ELISA Principle (Indirect ELISA)

Coat the well — antigen is bound to the surface of a microtitre plate well

Add sample — if the patient's sample contains antibodies against that antigen, they bind to the antigen on the well surface

Add secondary antibody — an enzyme-linked antibody that binds to the patient's antibody is added

Add substrate — the enzyme converts the substrate to a coloured product; colour develops only if the target antibody was present in the sample

Read results — a colour change indicates a positive result; colour intensity can quantify antibody concentration

Exam Tip

In the ELISA, it is the enzyme that produces the colour change — not the antibody directly. The antibody provides specificity; the enzyme provides the detectable signal. AQA may ask you to explain why a colour change occurs only in a positive sample.

ELISA — How It Works

Monoclonal Antibodies in Diagnosis — Pregnancy Test

The pregnancy test is the classic AQA example of monoclonal antibodies in medical diagnosis. It detects the hormone hCG (human chorionic gonadotropin) in urine.

How a Lateral Flow Pregnancy Test Works

Test strip contains two types of monoclonal antibody — both specific to hCG

Coloured antibodies (tagged with coloured particles) are free to move along the strip when urine is added. If hCG is present, it binds to these coloured antibodies

The hCG-antibody complexes travel along the strip to the test line, where fixed antibodies (also anti-hCG) capture them → the coloured particles accumulate → visible coloured line = positive result

Unbound coloured antibodies continue to a control line where they are captured regardless of the result — a coloured control line confirms the test is working

If no hCG is present: no colour at test line (negative); control line still appears

Synoptic Link

The same lateral flow principle is used in COVID-19 LFTs, detecting pathogen surface antigens rather than hormones. The fundamental mechanism — antibody specificity producing a detectable signal — is identical across all lateral flow applications.

Ethical Issues — Vaccines & Monoclonal Antibodies

AQA requires you to discuss ethical issues. Always present both sides, then reach a balanced conclusion.

Ethical Issues — Vaccines

Animal testing required in development — causes animal suffering

Side effects — rare but real; weighing individual risk against population benefit

Mandatory vaccination — does the state have the right to compel vaccination? Conflicts with bodily autonomy

Herd immunity and the unvaccinated — those who cannot be vaccinated (immune-compromised) depend on others' immunity; vaccine refusers may undermine this

Equity — access to vaccines is unequal between high- and low-income countries

Ethical Issues — Monoclonal Antibodies

Animal use — mice are immunised and used to produce hybridoma cells; raises animal welfare concerns

Cost — mAb therapies (e.g. cancer treatments) are extremely expensive; raises equity questions about NHS access and NICE decisions

Off-target effects — mAbs may bind to tissues expressing similar antigens, causing side effects in healthy tissue

Benefit vs. risk — for terminal cancer patients, severe side effects may be acceptable; this must be weighed case by case

Examiner Alert ⚠️

Ethics questions require both sides of an argument. A response listing only benefits, or only concerns, will not achieve full marks. AQA mark schemes award marks for identifying a genuine ethical concern AND acknowledging the countervailing benefit or justification.

DEEP Chain — Monoclonal Antibody Production

"Describe how monoclonal antibodies are produced." [5 marks]

DEEP Chain

DDefine: Monoclonal antibodies are identical antibodies produced by a single clone of hybridoma cells, all specific to one antigen.

EExplain mechanism: A mouse is immunised with the target antigen — its B cells produce antibodies specific to that antigen. These B cells are extracted from the spleen and fused with myeloma cells (immortal tumour cells) to form hybridoma cells.

EEvidence/named detail: Hybridoma cells are immortal (from the myeloma) AND produce the specific antibody (from the B cell). They are screened to identify those producing the desired antibody, then cultured in large quantities. All antibodies produced are identical — monoclonal — and specific to the original antigen.

PPractical/application: Monoclonal antibodies can be conjugated to cytotoxic drugs to target cancer cells specifically, or used in ELISA and lateral flow tests to detect the presence of specific antigens (e.g. hCG in pregnancy tests, pathogen antigens in disease diagnosis).

Practice Question 1

A patient with late-stage AIDS develops a severe lung infection caused by Pneumocystis jirovecii, a fungus that rarely causes disease in healthy people.

(a) Explain, in terms of the immune system, why this patient is susceptible to infections that would not affect a healthy person. [4 marks]

(b) The patient's doctor prescribes antifungal drugs but explains that antibiotics would not have been useful even if the infection were bacterial. Explain why antibiotics are ineffective against viruses such as HIV. [2 marks]

D:

HIV specifically infects and destroys helper T cells (T_H cells). As helper T cell numbers fall below the critical threshold, the specific immune response is severely impaired. [1]

E:

Helper T cells normally release cytokines that stimulate both cytotoxic T cells (cellular response) and B cells (humoral response → antibody production). Without T_H cells, neither arm of the specific immune response can function effectively. [2]

E:

With the specific immune response compromised, even pathogens with low virulence — such as P. jirovecii — can cause serious disease because they are not recognised and eliminated efficiently. [1]

P (b):

Antibiotics work by targeting bacterial-specific structures — primarily the murein (peptidoglycan) cell wall and 70S ribosomes. Viruses possess neither of these structures; they have no cell wall and no ribosomes of their own. Therefore antibiotics have no target in a viral infection and are ineffective against HIV. [2]

Practice Question 2

A pharmaceutical company wishes to produce a monoclonal antibody therapy that targets a protein antigen found only on the surface of breast cancer cells.

(a) Describe how the company would produce the monoclonal antibodies. [4 marks]

(b) The antibody is conjugated to a cytotoxic drug before being given to patients. Explain one advantage of this approach over conventional chemotherapy that treats all dividing cells. [2 marks]

(c) Identify one ethical issue associated with the production of this monoclonal antibody. [1 mark]

D (a):

A mouse is immunised with the breast cancer cell antigen. B cells that produce antibodies specific to this antigen are extracted from the mouse's spleen. [1]

E (a):

The B cells are fused with myeloma cells (immortal tumour cells) to produce hybridoma cells. These are immortal and produce the specific antibody. [1]

E (a):

Hybridomas are screened to identify those producing the correct antibody, then cloned and cultured to produce large quantities of identical (monoclonal) antibodies specific to the cancer antigen. [2]

P (b):

The monoclonal antibody binds specifically to cancer cells expressing the target antigen — the cytotoxic drug is delivered directly to these cells. Healthy cells that do not express the antigen are not exposed to the drug, minimising damage to normal tissue. Conventional chemotherapy attacks all rapidly dividing cells, causing widespread side effects such as hair loss and immune suppression. [2]

P (c):

Animals (mice) must be immunised and used to produce the hybridoma cells, raising animal welfare concerns. OR: The treatment may be very expensive, raising equity concerns about which patients can access it. [1]

Summary

Key Points — Spec 3.2.4 (Lesson 2.07)

TopicWhat to Know

HIV structureRNA retrovirus; lipid envelope; glycoprotein spikes (gp120); protein capsid; reverse transcriptase enzyme

HIV replicationgp120 binds CD4 on T_H → RNA + reverse transcriptase enter cell → DNA synthesised → integrates into host chromosome (provirus) → new virions bud off → T_H cell destroyed

AIDST_H count <200/µL; immune system compromised; opportunistic infections; antibiotics ineffective against HIV (no cell wall / no 70S ribosomes to target)

Monoclonal antibody productionImmunise mouse → extract B cells → fuse with myeloma cells → hybridoma → screen → clone → monoclonal antibodies

Uses of mAbsTargeted drug delivery (cancer); diagnosis (pregnancy test/hCG, lateral flow); ELISA

ELISAAntibody-enzyme conjugate detects antigen; enzyme converts substrate to coloured product; colour = positive

Ethics — vaccinesAnimal testing; side effects; individual vs. population benefit; equity of access; mandatory vaccination vs. autonomy

Ethics — mAbsAnimal use in production; high cost/NHS access; off-target effects

Key Distinction

Reverse transcriptase: RNA → DNA (this is the reverse of normal transcription DNA → RNA). This is why HIV is called a retrovirus.

Why mAbs Work

Specificity — each binds only one antigen. This makes them ideal for targeted delivery and precise diagnostic tests where non-specific binding would give false positives.

Top 3 Examiner Errors

1. "Antibiotics kill viruses" — antibiotics target bacterial structures absent from viruses.

2. "Monoclonal antibodies kill cancer cells" — the drug attached to the mAb kills the cell; the antibody provides targeting.

3. Forgetting that HIV reverse transcriptase converts RNA → DNA (not RNA → protein).