11.2.510-11 · Infectious Disease and Immunity

How Vaccines Work — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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11.2.5 How Vaccines Work — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how vaccines produce immunity, compare the types of vaccine and describe how they are tested.Definition — a vaccine is a suspension of antigens put into the body to induce artificial ac…

Primary response
Secondary response
Live attenuated
Inactivated and subunit
Antigenic variation
Many serotypes

Part 1 of 7The principle of vaccination

Definition

Definition a vaccine is a suspension of antigens put into the body to induce artificial active immunity.

Route

Route it is given by injection or orally.
- (into a vein or muscle)

Primary response

Primary response macrophages present the antigens, helper T cells are activated, B cells are selected and cloned, and plasma cells make antibodies.

Memory

Memory memory B and T cells form.

Secondary response

Secondary response on meeting the real pathogen it is so rapid and large that the pathogen is destroyed before symptoms develop.

Boosters

Boosters second or third doses weeks or months later raise memory cell numbers and antibody levels.

Part 2 of 7Types of vaccine

Type	Description	Examples
Live attenuated	Weakened living pathogen that multiplies slowly; strong, long-lasting response	MMR, BCG, oral polio (Sabin)
Inactivated (killed)	Dead whole pathogen; cannot replicate, so boosters are needed	Injected polio (Salk), hepatitis A
Subunit or toxoid	Purified antigen or inactivated toxin	Tetanus toxoid, hepatitis B, HPV
Nucleic acid	mRNA or DNA coding for an antigen, made by host cells	mRNA vaccines for COVID-19

Types of vaccine

Live attenuated

Live attenuated not suitable for people with weak immune systems, since the weakened pathogen may cause disease.

Inactivated and subunit

Inactivated and subunit they are safer but often need adjuvants and several doses.

Part 3 of 7Why vaccines are not always effective

Why vaccines are not always effective

Antigenic variation

Antigenic variation influenza changes its antigens,
so the vaccine is updated annually;
- HIV and malaria change theirs so quickly that effective vaccines have been hard to make.

Many serotypes

Many serotypes some pathogens, such as common cold viruses, have many strains.

Hiding pathogens

Hiding pathogens viruses inside cells, or HIV infecting immune cells, avoid antibody detection.

Weak memory

Weak memory vaccines with small antigens may give a weak or short-lived response.

Individual variation

Individual variation older people and those with weak immune systems may respond poorly.

Practical issues

Practical issues some need a cold chain and trained staff, and cost may limit use.

Side effects

Side effects most are mild and serious allergic reactions are very rare.
- (soreness, fever)

Part 4 of 7Data and evaluation

Data and evaluation

Incidence data

Incidence data describe the trend with figures, calculate the percentage fall and explain that vaccination gave immunity to most of the population.

Rises in cases

Rises in cases consider falling vaccination rates, a new strain or waning immunity.

Evidence

Evidence correlation is not proof, but a clear drop soon after introduction with no change in other factors strongly supports effectiveness.

Comparing types

Comparing types relate vaccine safety to the strength of immune response.

Part 5 of 7Designing and testing a vaccine

Designing and testing a vaccine

Preclinical studies test candidates in cells and animals.

Phase 1 tests safety in a small group of volunteers.

Phase 2 tests the immune response and dose in hundreds.

Phase 3 compares the vaccine with a placebo in thousands of people to measure how many develop the disease.

The vaccine is licensed.

Continuing surveillance monitors rare side effects.

Trial design

Trial design trials are usually randomised and double-blind,
- so neither volunteers nor researchers know who received which treatment,
- which prevents bias.

Placebo and large samples

Placebo and large samples they give valid evidence.

Vaccine efficacy

(risk in unvaccinated − risk in vaccinated) ÷ risk in unvaccinated × 100, a proportional reduction in disease.

Part 6 of 7More from the specification

1Toxoid vaccines neutralizing bacterial toxins

Toxoid vaccines neutralizing bacterial toxins administering inactivated microbial toxins stimulates plasma cells to produce protective antitoxin antibodies.

2Monoclonal antibody production via hybridomas

Hybridoma cells formed by fusing B-lymphocytes with myeloma cells produce identical monoclonal antibodies specific to a single target epitope.

3Diagnostic and therapeutic applications

Monoclonal antibodies serve as precise diagnostic tools to detect tumour markers or pathogens in blood,
and can deliver cytotoxic drugs directly to cancer cells.

4Passive immunity vs active immunity

Passive immunity provides immediate but temporary protection through pre-formed antibodies without memory cells,
whereas active immunity stimulates lasting endogenous memory cell production.

5Monoclonal antibody production via hybridomas

Hybridoma cells formed by fusing B-lymphocytes with myeloma cells produce identical monoclonal antibodies specific to a single target epitope.

6Diagnostic and therapeutic applications

Monoclonal antibodies serve as precise diagnostic tools to detect tumour markers or pathogens in blood,
and can deliver cytotoxic drugs directly to cancer cells.

7Passive immunity vs active immunity

Passive immunity provides immediate but temporary protection through pre-formed antibodies without memory cells,
whereas active immunity stimulates lasting endogenous memory cell production.

8Monoclonal antibody

Identical antibodies from one clone, all binding the same epitope.

9Passive immunity

Short-term immunity from antibodies given by another individual.

10Variable region

The arm tip that forms the antigen-binding site.

11Receptor protein

Protein on the postsynaptic membrane that binds neurotransmitter.

12Specific

Able to act on only one type of substrate.

13Hybridoma

A fused plasma cell and tumour cell that makes one antibody.

14Cancer

Disease in which cells divide uncontrollably and spread.

15Protein

A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

16Tumour

A lump of cells formed by uncontrolled division.

Part 7 of 7More on this topic

Term	Meaning
Chemotaxis	Movement along a chemical gradient.
Cytokine	A signalling chemical released by T helper cells.
Clonal selection	Selection of the lymphocyte with the matching receptor.
Hybridoma	A fused plasma cell and tumour cell that makes one antibody.
Toxoid	An inactivated toxin used in vaccines.
Herd immunity	Protection of a population when most are immune.

Key terms

Term	Meaning
Vaccine	A suspension of antigens that induces artificial active immunity.
Booster	A later dose that raises memory cell numbers and antibody levels.
Live attenuated vaccine	A vaccine of weakened live pathogens.
Toxoid	An inactivated, harmless form of a toxin.
Adjuvant	A substance that boosts the immune response to a vaccine.
Vaccine efficacy	The proportional reduction in disease among the vaccinated.

Avoid these mistakes

Saying a vaccine contains active disease-causing pathogens — they are weakened, killed or partial.

Saying a vaccine gives immediate protection — it needs a primary response first.

Saying all vaccines last for life — inactivated ones need boosters.

Saying herd immunity makes vaccination unnecessary for individuals — it relies on high coverage.

Last updated: 15 October 2026