Vaccination, Monoclonal Antibodies and ELISA

Spec mapping: AQA 7402 Section 3.2.4 — vaccination, monoclonal antibodies and the ELISA test (refer to the official AQA specification document for exact wording).

Vaccination is arguably the single most consequential medical innovation of the modern era. The eradication of smallpox (declared by the WHO in 1980), the near-eradication of poliomyelitis, the dramatic decline of childhood measles mortality, and the rapid development of COVID-19 vaccines have collectively saved hundreds of millions of lives. Monoclonal antibodies (mAbs) are a related triumph — engineered antibody molecules of a single specificity, manufactured at scale, now used as targeted therapeutics for cancer, autoimmunity, infectious disease and inflammation. ELISA, introduced in lesson 8, is the diagnostic workhorse that links both. This lesson covers active vs passive immunity, herd immunity, the major vaccine types, monoclonal antibody production by the hybridoma technique, therapeutic mAbs, ELISA in clinical context, and the ethical questions surrounding modern immune-based medicine.

Key Definition: A vaccine is a biological preparation that induces adaptive immunity to a specific pathogen without causing the disease itself. The mechanism is to expose the immune system to a controlled antigen so that a primary response — with memory-cell generation — occurs in advance of natural exposure, ensuring that the natural exposure (if it ever occurs) triggers a secondary response.

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Active vs Passive Immunity

Adaptive immunity can be acquired in two fundamentally different ways. The four-way matrix below is the AQA-style framework you must master.

	Natural	Artificial
Active (own immune system generates response + memory)	Recovery from natural infection (e.g., measles → lifelong immunity)	Vaccination (e.g., MMR vaccine)
Passive (pre-formed antibody transferred; no memory)	Maternal antibodies (IgG across placenta; IgA in breast milk)	Therapeutic antibody injection (e.g., anti-tetanus immunoglobulin after a deep wound; rabies post-exposure prophylaxis)

Active immunity

• Generates memory B and T cells.
• Slow to develop (weeks for primary response, but long-lasting — sometimes lifelong).
• Cell-mediated and humoral.
• Either naturally acquired (after surviving infection) or artificially induced (vaccination).

Passive immunity

• No memory cells are formed because the recipient's own B and T cells were not activated.
• Fast: protection begins immediately on antibody transfer.
• Short-lived: transferred antibodies decay over weeks to months.
• Critical in scenarios where the patient has no time to mount their own response (post-exposure prophylaxis, neonatal protection).

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Herd Immunity

When a sufficiently high proportion of a population is immune (whether by vaccination or recovery), pathogen transmission is interrupted because each infected individual encounters too few susceptible contacts to sustain a chain of infection. This effect protects unvaccinated individuals — newborns, the immunocompromised, those with vaccine contraindications — by drastically reducing their probability of exposure. This is herd immunity.

Basic Reproduction Number (R₀)

R₀ is the average number of secondary infections produced by one infected individual in a fully susceptible population.

• R₀ < 1: epidemic dies out.
• R₀ = 1: stable endemic disease.
• R₀ > 1: epidemic spreads.

Herd Immunity Threshold

The proportion of the population that must be immune to drive effective R below 1 is approximately:

H_T = 1 − 1/R₀

Disease	R₀ (approximate)	H_T
Measles	12–18	~93–95%
Pertussis (whooping cough)	12–17	~92–94%
Mumps	4–7	~75–86%
Polio	5–7	~80–86%
Influenza (seasonal)	1.5–2.5	~33–60%

These thresholds explain why the WHO targets >95% MMR coverage: anything materially lower allows measles outbreaks to recur, as has been observed when vaccination rates fall.

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Vaccine Types

Different vaccine technologies trade off immunogenicity, safety and cost.

Live attenuated vaccines

• Contain weakened (avirulent) live pathogens. Examples: MMR (measles, mumps, rubella), BCG (tuberculosis), yellow fever, oral polio (Sabin).
• Highly immunogenic — induce strong, durable immunity, often lifelong.
• Drawback: cannot be given to severely immunocompromised individuals (risk of disease from the vaccine strain).
• Require cold-chain storage.

Killed/inactivated vaccines

• Contain whole pathogen killed by heat, chemicals or radiation. Examples: inactivated polio (Salk), hepatitis A, rabies.
• Safer for immunocompromised — cannot replicate.
• Generally less immunogenic; often require multiple doses and adjuvants.

Subunit vaccines

• Contain specific purified pathogen molecules (proteins, polysaccharides) rather than whole pathogen. Examples: hepatitis B (surface protein), HPV (capsid proteins), pneumococcal polysaccharide vaccines.
• Very safe; defined composition; no risk of disease.
• Often less immunogenic — require adjuvants and booster doses.

Toxoid vaccines

• Contain inactivated bacterial toxins (toxoids) — induce antibodies that neutralise the natural toxin. Examples: tetanus, diphtheria.

mRNA vaccines

• Contain mRNA encoding a pathogen antigen, packaged in lipid nanoparticles. Recipient cells translate the mRNA, producing the antigen which then triggers immunity. Example: COVID-19 vaccines (Pfizer–BioNTech, Moderna).
• Fast to design and manufacture; no live pathogen; can be updated for new variants.
• Cold-chain requirements were initially demanding.

Viral vector vaccines

• Use a harmless virus (e.g., adenovirus) engineered to carry pathogen antigen genes. Recipient cells produce the antigen. Example: Oxford–AstraZeneca COVID-19 vaccine.

Historical context — paraphrased

Edward Jenner's late-18th-century observation that milkmaids exposed to cowpox were protected from smallpox provided the empirical framework for vaccination — the term itself derives from vacca, cow. Louis Pasteur extended the principle with attenuated rabies and anthrax vaccines. Robert Koch's framework for establishing pathogens as causes of disease (paraphrased here as Koch's postulates: associated with disease; isolatable; reproducible in a new host; re-isolatable) created the conceptual foundation for targeted vaccines. Jonas Salk's inactivated polio vaccine (1955) and Albert Sabin's oral live-attenuated vaccine (1961) drove polio toward near-eradication. The smallpox eradication campaign concluded successfully in 1980 — the first and so far only human disease eliminated by vaccination.

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Monoclonal Antibody Production — Hybridoma Technology

Monoclonal antibodies (mAbs) are antibodies of a single defined specificity, all identical, derived from a single B-cell clone. They differ from polyclonal antibodies (mixed antibodies from many B-cell clones, each binding a different epitope) in providing reproducible, single-specificity reagents — essential for diagnostics, research and therapeutics.

The Hybridoma Technique — Paraphrased Historical Framework

The hybridoma technique was developed in the mid-1970s by Georges Köhler and César Milstein at the MRC Laboratory of Molecular Biology in Cambridge — work that won the 1984 Nobel Prize in Physiology or Medicine. Their conceptual breakthrough was to fuse a normal antibody-producing B cell (mortal but specific) with an immortal myeloma cancer cell (immortal but irrelevant), producing a hybridoma cell with both desired properties: specificity from the B cell and immortality from the myeloma. The technical implementation paraphrased:

1. Immunise an animal (typically a mouse) with the target antigen. After several weeks, the spleen contains B cells producing antibodies against the antigen.
2. Harvest spleen cells (containing a mixed population of B cells, some specific for the antigen).
3. Fuse spleen cells with myeloma cells using polyethylene glycol (PEG), which promotes cell membrane fusion. Some fusion events produce hybridomas — cells with the B-cell antibody machinery and the myeloma's immortal proliferation.
4. Select hybridomas using HAT medium (hypoxanthine, aminopterin, thymidine). Unfused myeloma cells lack the salvage-pathway enzyme HGPRT and die in HAT. Unfused B cells die naturally in culture. Only hybridomas (with HGPRT from the B-cell parent) survive.
5. Screen surviving hybridomas for antibody binding to the antigen (typically by ELISA).
6. Clone the chosen hybridoma by limiting dilution to ensure all cells derive from one founder, so all antibodies are identical (monoclonal).
7. Scale up clone in bioreactors. Antibody is harvested and purified from culture supernatant.

Humanisation

Early mAbs were entirely murine (mouse), causing HAMA (human anti-mouse antibody) responses in patients — the body's immune system attacking the mouse-derived therapeutic. Modern therapeutic mAbs are humanised (mouse variable regions grafted onto human constant regions, or fully human (produced from human B cells or transgenic mice with humanised antibody gene loci). This has dramatically improved tolerability.

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Therapeutic Monoclonal Antibodies

Therapeutic mAbs now constitute the largest category of biologic drugs by value. Their specificity allows precise targeting that small-molecule drugs typically cannot achieve.

Examples and Categories

• Trastuzumab — binds the HER2 (human epidermal growth factor receptor 2) receptor on HER2-positive breast cancer cells, blocking growth signalling and triggering immune destruction. Used in HER2+ breast cancer as targeted therapy.
• Rituximab — binds CD20 on B cells, depleting them in B-cell lymphomas and autoimmune diseases (rheumatoid arthritis, multiple sclerosis).
• Adalimumab and infliximab — bind tumour necrosis factor (TNF), blocking inflammatory signalling in rheumatoid arthritis, Crohn's disease, psoriasis.
• Pembrolizumab and nivolumab — checkpoint inhibitors, binding PD-1 to release T-cell suppression of tumours; revolutionised treatment of advanced melanoma and lung cancers.
• Bevacizumab — binds VEGF (vascular endothelial growth factor), blocking tumour angiogenesis.
• Palivizumab — binds respiratory syncytial virus (RSV) protein, used as passive immunoprophylaxis in vulnerable infants.

The clinical impact has been substantial across cancer, autoimmunity, infectious disease and inflammation. mAbs do not, however, cure all targets they engage; mechanisms of resistance (target downregulation, antigen variation, immune evasion) recur.

Mechanism of Action Categories

• Neutralisation: block a target's binding to its physiological partner (e.g., adalimumab blocking TNF).
• Receptor blockade: prevent signalling (e.g., trastuzumab on HER2).
• Antibody-drug conjugates: chemically link mAb to a cytotoxic drug; mAb delivers the toxin to target cells (e.g., trastuzumab emtansine).
• Immune cell recruitment: Fc region engages NK cells (ADCC) or complement (CDC) to destroy target.
• Checkpoint inhibition: block immune checkpoint molecules, unleashing T cells against tumours.

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ELISA in Diagnostic Context

ELISA introduced in lesson 8 finds its largest application in clinical diagnostics. Two iconic uses:

HIV testing

The fourth-generation HIV combination assay is a sandwich ELISA detecting both HIV p24 antigen (present early) and anti-HIV antibodies (present after seroconversion, typically 3–12 weeks). This combination shortens the diagnostic window. Initial reactive ELISA results are confirmed by Western blot or PCR.

COVID-19 serology

ELISA-based assays detect antibodies against SARS-CoV-2 spike or nucleocapsid proteins, indicating prior infection or vaccination. Sandwich ELISA quantifies antibody titre; this informs vaccine effectiveness studies and population seroprevalence surveys.

Pregnancy tests

Home pregnancy tests use a lateral-flow variant of sandwich ELISA detecting β-hCG (human chorionic gonadotropin) in urine, exploiting the same antibody-antigen specificity but in a strip format rather than a microtitre plate.

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Mermaid Diagram — Hybridoma Production

flowchart TD
  Im[Immunise mouse with antigen] --> Sp[Harvest spleen B cells]
  My[Myeloma cells in culture] --> Fuse[Fuse with PEG]
  Sp --> Fuse
  Fuse --> HAT[Select in HAT medium]
  HAT --> Screen[Screen for antigen binding by ELISA]
  Screen --> Clone[Clone by limiting dilution]
  Clone --> Scale[Scale up hybridoma]
  Scale --> Ab[Harvest monoclonal antibody]

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Ethical Considerations

Immune-based medicine raises substantial ethical issues that A-Level Biology routinely examines as AO3 evaluation.

Animal use in monoclonal antibody production

• Mice (and to a lesser extent rabbits) are immunised and have their spleen harvested for hybridoma production. Although individual animal numbers are small per mAb, the cumulative scale is significant.
• Recombinant alternatives — phage display libraries, single-B-cell sequencing — increasingly allow fully human mAbs to be developed without animal immunisation. These are gradually displacing classical hybridoma methods.
• The principle of the 3Rs (replacement, reduction, refinement) applies: where alternatives exist, they should be preferred.

Vaccine hesitancy

• A historically common topic on AQA papers. Reasons for vaccine hesitancy include: misunderstanding of risk (overestimating rare adverse events, underestimating disease burden), distrust of pharmaceutical and government institutions, religious or philosophical objections, and (historically influential but now retracted) the fraudulent 1998 paper linking MMR with autism. The paper was retracted; the alleged link is unsupported by extensive subsequent evidence — but the public health damage was considerable.
• Public-health responses include transparent risk communication, accessible vaccination services, and trusted local community channels.

Equitable access

• COVID-19 vaccination starkly exposed global inequities: high-income countries achieved early high coverage while low- and middle-income countries waited months or years for adequate supply.
• Monoclonal antibody therapies are typically very expensive (initial therapy courses can exceed £100,000) and disproportionately available in wealthy health systems.
• Patent and licensing decisions, technology transfer, and global manufacturing capacity all shape access.

Informed consent and population-level coverage