How does vaccination produce immunity without illness, and how does herd immunity protect a population?
Explain how vaccination produces active immunity and how herd immunity protects a population, and distinguish active and passive immunity
A focused answer to the H2 Biology Infectious Disease and Immunity outcome on vaccination. How a vaccine produces active immunity through the primary response and memory cells, the difference between active and passive immunity, and how herd immunity protects a population.
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What this dot point is asking
SEAB wants you to explain how vaccination produces active immunity (through a primary response and memory cells) without causing illness, to distinguish active from passive immunity, and to explain herd immunity. This applies the immune response, antibodies and memory to public health.
The answer
How a vaccine works
A vaccine contains antigens from a pathogen, for example a weakened or dead form, or just the antigen molecules. These are recognised as foreign but cannot cause the disease.
The antigens trigger a primary immune response: the specific lymphocytes are selected and expanded, plasma cells make antibody, and crucially memory cells are produced and persist. Because the antigen is not virulent, the person gains the memory without the illness. On later exposure to the real pathogen, the memory cells give a rapid, large secondary response that destroys it before symptoms develop.
Active versus passive immunity
- Active immunity: the person's own immune system responds and makes its own antibodies and memory cells. It develops slowly but is long-lasting. Examples: recovering from infection (natural), or vaccination (artificial).
- Passive immunity: ready-made antibodies are received from another source. It is immediate but short-lived, with no memory cells. Examples: antibodies from mother to baby across the placenta or in breast milk (natural), or an antibody injection (artificial).
Herd immunity
When a large enough proportion of a population is immune (usually through vaccination), the pathogen cannot spread easily because it rarely meets a susceptible person. This herd immunity protects even those who are not immune, such as the very young or those who cannot be vaccinated, by interrupting transmission.
Examples in context
Example 1. Vaccination programmes and eradication. Widespread vaccination can reduce a pathogen to the point where it can no longer find enough susceptible hosts, and a disease can in principle be eliminated from a region. This is herd immunity applied at national scale.
Example 2. Antibody injections for immediate protection. Someone exposed to a dangerous toxin or pathogen may be given ready-made antibodies for instant protection while their own response is too slow to help. This passive immunity buys time but must be followed by active immunisation for lasting protection.
Try this
Q1. State what a vaccine contains that triggers the immune response. [1 mark]
- Cue. Antigens from the pathogen (for example a weakened or dead form, or the antigen molecules).
Q2. Explain why active immunity lasts longer than passive immunity. [2 marks]
- Cue. Active immunity produces the person's own memory cells, which persist and give a rapid response on re-exposure; passive immunity provides only received antibodies, which are broken down and leave no memory.
Q3. Explain how herd immunity protects a person who has not been vaccinated. [2 marks]
- Cue. When most of the population is immune, the pathogen rarely meets a susceptible person, so it cannot spread, and the unvaccinated person is unlikely to be exposed.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksExplain how a vaccine produces long-lasting immunity to a disease without the person becoming ill.Show worked answer →
The answer should connect the vaccine to the primary response and memory.
A vaccine contains antigens from the pathogen, for example a weakened or dead form, or just the antigen molecules. These antigens are recognised as foreign by the immune system but cannot cause the disease.
The antigens trigger a primary immune response: the specific B and T lymphocytes are selected and undergo clonal expansion. Plasma cells make antibodies, and importantly memory cells are produced and remain in the body.
Because the antigens cannot cause illness, the person gains the memory without the disease. If they later meet the real pathogen, the memory cells produce a rapid, large secondary response that destroys the pathogen before symptoms develop, so the person is immune.
Markers reward the vaccine containing harmless antigens, the primary response producing memory cells, no illness because the antigen is not virulent, and the rapid secondary response on later exposure giving immunity.
Original4 marksDistinguish between active and passive immunity, giving an example of each.Show worked answer →
The answer should contrast how the antibodies are obtained and the duration.
Active immunity is gained when a person's own immune system responds to an antigen and makes its own antibodies and memory cells. It develops more slowly but is long-lasting because memory cells are produced. Examples are recovering from an infection (natural) or receiving a vaccine (artificial).
Passive immunity is gained when ready-made antibodies are received from another source rather than being produced by the person's own immune system. It acts immediately but is short-lived, because no memory cells are made and the antibodies are eventually broken down. Examples are antibodies passed from mother to baby across the placenta or in breast milk (natural) or an injection of antibodies (artificial).
Markers reward active immunity involving the person's own response and memory cells (long-lasting), passive immunity involving received antibodies (immediate but short-lived, no memory), and a correct example of each.
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