Immunity, HIV, Monoclonal Antibodies, and Ethical Considerations (Part 1/2) Revision Notes | A Level AQA Biology

1. Why Your Immune System Has a Memory

Think about why you only get chickenpox once. The first time the varicella-zoster virus enters your body, you feel ill for days while your immune system scrambles to respond. The second time the same virus arrives, you barely notice — your body destroys it before symptoms appear. That difference comes down to two cell types: plasma cells and memory cells.

When a B lymphocyte encounters its complementary antigen, it undergoes clonal expansion — dividing rapidly to produce a large clone of identical cells. Most of these differentiate into plasma cells, which are essentially antibody factories, secreting thousands of specific antibody molecules per second. A smaller proportion differentiate into memory B cells, which persist in the body for years or even decades.

During the primary response, there is a lag phase of several days before antibody levels rise detectably in the blood. This is the window during which you feel ill — the pathogen is replicating faster than your immune system can neutralise it. Helper T cells (CD4+ T cells) play a crucial role here: they release cytokines that stimulate B cell activation and proliferation, amplifying the response.

Memory cells are the biological basis of immunological memory. Because they are already antigen-specific and present in large numbers, the lag phase is dramatically shortened in the secondary response. Antibody titres rise more steeply, reach a higher peak, and remain elevated for longer. This is precisely why vaccines work — they prime memory cells without causing disease.

The spec requires you to link plasma cells and memory cells explicitly to both the primary and secondary responses. A common exam error is to describe memory cells as 'producing antibodies directly' — they do not. Memory cells must first differentiate into plasma cells before antibody secretion occurs.

2. Vaccines, Active Immunity, and Herd Protection

Now that you understand why memory cells matter, you can see exactly why vaccination is so powerful. A vaccine introduces an antigen — or something that mimics one — without causing disease. The immune system mounts a primary response, memory cells form, and the person is protected against future infection. This is active immunity.

Contrast this with passive immunity, where ready-made antibodies are transferred into a person rather than being produced by their own immune system. A newborn baby receives maternal IgG antibodies across the placenta and through breast milk — this is natural passive immunity. Clinically, antitoxins or antibody preparations can be injected to provide immediate but temporary protection (for example, after a snake bite or tetanus exposure).

The key distinction examiners test is whether memory cells are produced. Passive immunity gives no memory cells, so protection ends when the transferred antibodies are degraded. Active immunity produces memory cells, giving durable protection.

Vaccines protect not only the individual but also the wider population through herd immunity. When enough people in a community are immune, a pathogen cannot spread efficiently — even unvaccinated individuals (such as newborns or immunocompromised patients) gain indirect protection because the chain of transmission is broken. The proportion of the population that must be immune to achieve herd immunity varies by pathogen; for measles, which is highly transmissible, it is approximately 95%.

3. HIV Structure and Replication in Helper T Cells

With immunity established as a concept, we can now examine what happens when the immune system itself is targeted. The human immunodeficiency virus (HIV) is a retrovirus that specifically infects helper T cells (CD4+ T cells) — the very cells that coordinate both humoral and cell-mediated immunity. Understanding its structure explains how it hijacks the cell.

• Outer lipid envelope — derived from the host cell membrane when the virus buds out; contains glycoprotein spikes (gp120) that bind to CD4 receptors on helper T cells
• Capsid (protein coat) — encloses the genetic material and enzymes
• Two single-stranded RNA molecules — the viral genome (HIV is an RNA virus)
• Reverse transcriptase enzyme — converts viral RNA into DNA inside the host cell
• Integrase enzyme — inserts the viral DNA into the host cell's chromosomes

The glycoprotein spikes are the key to specificity: gp120 has a complementary shape to the CD4 receptor on helper T cells. This is why HIV targets helper T cells almost exclusively. Once bound, the viral envelope fuses with the host cell membrane and the capsid contents enter the cytoplasm.

Worked example: HIV Replication in a Helper T Cell

Trace the sequence of events from HIV binding to a helper T cell through to the release of new virus particles.

1. Viral gp120 glycoprotein binds to the CD4 receptor on the surface of a helper T cell. — The complementary shape between gp120 and CD4 determines host-cell specificity. This is why HIV does not infect most other cell types.
2. The viral lipid envelope fuses with the host cell membrane; the capsid and its contents (RNA + enzymes) enter the cytoplasm. — The lipid envelope is essential for this fusion step — it is one reason enveloped viruses behave differently from non-enveloped viruses.
3. Reverse transcriptase converts the single-stranded viral RNA into double-stranded DNA (proviral DNA). — This step is unique to retroviruses. The enzyme is error-prone, generating mutations that contribute to HIV's rapid evolution and drug resistance.
4. Integrase inserts the proviral DNA into the host cell's chromosomes. — Once integrated, the provirus can remain latent for years. This is why HIV cannot be fully eliminated by the immune system or current antiretroviral drugs.
5. Host cell RNA polymerase transcribes the proviral DNA; host ribosomes translate viral mRNA into viral proteins. — The virus exploits the host cell's own molecular machinery — it contributes no ribosomes of its own.
6. New viral RNA and proteins assemble near the cell membrane; new virus particles bud off, acquiring a lipid envelope from the host membrane. — Budding eventually destroys the helper T cell. As helper T cell numbers decline, immune coordination collapses.

Answer: HIV binds via gp120 to CD4 receptors, fuses with the membrane, reverse-transcribes its RNA into DNA, integrates into the host genome, uses host machinery to produce viral components, then buds out — destroying the helper T cell in the process.

Integrase inserts proviral DNA into the host chromosome, where it can be transcribed using host machinery to produce new viral components, and may also establish latency.

The infections and cancers that develop as a result — such as Pneumocystis pneumonia or Kaposi's sarcoma — are called opportunistic infections. They are caused by pathogens that are normally harmless to immunocompetent individuals. AIDS is therefore not caused directly by HIV replication but by the immunodeficiency that results from the loss of helper T cells.

This point about antibiotics is a reliable exam target. The correct explanation must identify a specific bacterial structure or process that antibiotics act on, and then state that viruses lack that structure or process. Saying 'antibiotics only kill bacteria' earns one mark; explaining why — because viruses use host cell machinery and lack bacterial cell walls or ribosomes — earns the additional mark.

4. Monoclonal Antibodies: Precision Tools in Medicine

Earlier in this lesson, we saw how the immune system generates antibodies with exquisite specificity for a single antigen. Monoclonal antibodies (mAbs) harness that specificity artificially. Because every molecule in a monoclonal antibody preparation is identical and targets the same epitope, they can be used as precise molecular tools — in diagnosis, in therapy, and in research.

The AQA specification does not require you to know the commercial or scientific production method for monoclonal antibodies. What you must know is how they are used: in medical diagnosis, in the ELISA test, and in targeted drug therapy.

4a. Targeted Drug Therapy

One of the most clinically significant applications is attaching a therapeutic drug to a monoclonal antibody to deliver it specifically to target cells. Picture a cancer cell that overexpresses a particular surface protein. A monoclonal antibody specific to that protein will bind only to cells carrying it, delivering a cytotoxic drug directly to the tumour while leaving healthy cells unharmed.

This approach — sometimes called an antibody-drug conjugate — reduces the systemic side effects associated with conventional chemotherapy, where the drug circulates throughout the body and damages rapidly dividing healthy cells (such as those in the gut lining or hair follicles) as well as tumour cells. The antibody acts as a targeting vehicle; the drug is the payload.

4b. Medical Diagnosis and the ELISA Test

Monoclonal antibodies are also central to medical diagnosis. Because they bind specifically to a single antigen, they can detect the presence of a target molecule — a hormone, a pathogen protein, or a cancer marker — even in very small quantities. Pregnancy tests, for example, use monoclonal antibodies specific to human chorionic gonadotrophin (hCG).

The ELISA test (enzyme-linked immunosorbent assay) is the most important diagnostic application you need to know for AQA. It uses antibodies linked to an enzyme to detect and quantify a specific antigen in a sample. A colour change produced by the enzyme's substrate indicates a positive result.

Worked example: How the ELISA Test Works

Describe the sequence of steps in an ELISA test used to detect a specific antigen in a patient's blood sample.

1. A primary antibody specific to the target antigen is attached to the surface of a microplate well. — This antibody acts as a 'capture' antibody. Its specificity ensures only the target antigen will be retained in subsequent steps.
2. The patient's sample (e.g. blood serum) is added to the well. If the target antigen is present, it binds to the primary antibody. — Unbound molecules are washed away after incubation, so only the antigen of interest remains.
3. A secondary antibody — also specific to the target antigen and conjugated to an enzyme — is added to the well. — The secondary antibody binds to the antigen that is already captured by the primary antibody, forming a 'sandwich'. The enzyme label is what will generate the detectable signal.
4. Unbound secondary antibody is washed away. — This step is critical for reducing background signal and ensuring the colour change is proportional to the amount of antigen present.
5. A substrate for the enzyme is added. If the secondary antibody (and therefore the antigen) is present, the enzyme converts the substrate, producing a colour change. — A positive result (colour change) indicates the antigen was present in the sample. The intensity of the colour can be measured to quantify the antigen concentration.

Answer: A colour change in the well indicates a positive result — the target antigen was present in the sample. No colour change indicates a negative result.

ELISA tests are used to diagnose HIV infection (detecting HIV antibodies or viral proteins in blood), to screen for certain cancers using tumour markers, and to detect hormones such as hCG in pregnancy tests. The principle is always the same: antibody specificity converts the presence of a target molecule into a detectable signal.

5. Exam Technique and Mark-Winning Strategies

The topics in this lesson appear frequently in AQA A Level Biology papers, and the marks are often lost not through lack of knowledge but through imprecise language. The following guidance targets the most common errors seen in mark schemes.

A reliable way to structure a 6-mark question on the immune response is to follow this sequence: name the cell type involved, describe what it does, explain the consequence for the organism, and link back to the question's context. For example, a question asking you to evaluate the use of monoclonal antibodies in cancer treatment should include: the mechanism of targeting (antibody specificity for tumour antigen), the benefit (reduced damage to healthy cells), a limitation (potential immune reactions to the antibody itself or high production costs), and a conclusion weighing these points.

The enzyme linked to the secondary antibody catalyses a reaction with the added substrate, producing a colour change that allows quantification of antigen concentration.