Immunity and vaccinationEdexcel A-Level Biology B: Revision notes
Section 1
Types of immunity
Immunity is the ability of the body to resist an infection by recognising and destroying a pathogen. It is classified in two ways.
- Active immunity: the person's own immune system makes antibodies and memory cells. It is slow to start but long-lasting.
- Passive immunity: the person receives ready-made antibodies and makes no antibodies or memory cells of their own. It acts immediately but is short-lived because antibodies are proteins that are broken down.
Immunity can also be natural (acquired through normal life events) or artificial (acquired through a medical procedure).
Do not say passive immunity 'does not work'. It works at once; it is just short-lived because no memory cells are made.
Section 2
Natural and artificial, active and passive
Combining the two ideas gives four types:
- Natural active: infection with a pathogen, after which the person makes antibodies and memory cells.
- Natural passive: antibodies from the mother cross the placenta or pass to the baby in breast milk (colostrum).
- Artificial active: vaccination, in which antigens from a harmless form of the pathogen are injected or swallowed, stimulating the person's own immune response.
- Artificial passive: injection of ready-made antibodies, for example against rabies or tetanus after exposure.
Ask two questions: did the person make the antibodies (active) or receive them (passive)? Was it a medical procedure (artificial) or a normal event (natural)?
Section 3
How vaccination produces protection
A vaccine contains antigens from the pathogen in a safe form, such as a weakened or inactivated (killed) pathogen, or purified antigens. The antigens are recognised as foreign:
- A B cell with a complementary receptor binds to the antigen and, with help from T helper cells, is activated (clonal selection).
- The B cell divides by mitosis (clonal expansion) and differentiates into plasma cells, which secrete antibodies, and memory cells.
- If the real pathogen is met later, memory cells give a secondary response that is faster and produces more antibody, so the pathogen is destroyed before symptoms develop.
Booster doses can be needed to keep enough memory cells.
Section 4
Vaccination and the control of disease
Vaccination programmes reduce the number of people who can be infected. Smallpox was eradicated by vaccination, and diseases such as polio and measles have been greatly reduced.
When enough of a population is immune, those who are not immune are protected because the pathogen cannot spread easily. This is herd immunity. The proportion needed depends on how infectious the disease is: for highly infectious measles it is about 95%.
Herd immunity matters because some people cannot be vaccinated, such as young babies, people with weakened immune systems and those having chemotherapy.
Section 5
When some people choose not to vaccinate
If the proportion vaccinated falls below the level needed for herd immunity, there are more susceptible people and the chain of transmission is no longer broken. The consequences are:
- Outbreaks of diseases that had been controlled, such as measles.
- Greater risk to babies, older people and immunosuppressed people who cannot be vaccinated and relied on herd immunity.
- Pressure on health services and, for diseases that can mutate, a larger pool in which the pathogen can evolve.
People may decline vaccination because of fear of side effects, religious or ethical beliefs, or misinformation. Side effects are generally mild and rare, but the individual's freedom of choice has to be balanced against the risk to others.
Herd immunity does not mean every person is immune. It means enough are immune that the pathogen cannot spread widely.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Immunity and vaccination
- A health visitor explains to new parents that their newborn baby is protected against several infections for the first few months of life. The protection comes from antibodies that crossed the placenta from the mother's blood before birth and from antibodies present in her breast milk.Explain why the protection from the mother's antibodies lasts only a few months.2 marks
- A school nurse is reviewing uptake of the MMR vaccine, which protects against measles, mumps and rubella. Measles is highly infectious and about 95% of a population must be immune to prevent outbreaks. In one town, 78% of children have been vaccinated. A small number of pupils cannot be vaccinated because they are receiving chemotherapy.Explain how a high level of vaccination in a town would protect the pupils who cannot be vaccinated.2 marks
- A hiker is bitten by a stray dog in a region where rabies occurs. The rabies virus travels slowly along nerves and can take several weeks to reach the brain, after which the disease is almost always fatal. Doctors give the hiker an injection of human rabies antibodies straight away, followed by a course of rabies vaccine over the next few weeks.Explain why the hiker is given both the rabies antibodies and the rabies vaccine.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).