2.6.1Cell Structure
Vaccines — AQA A Level Biology 7402 Revision Notes & Topical Questions
5 min read 9 parts Lesson 37 of 47 1 quiz
Lesson summary
2.6.1 Vaccines — AQA A Level Biology 7402 notes: Explain how vaccination provides protection, including herd immunity, and why some diseases are difficult to control by vaccination.A vaccine contains one or more antigens from a pathogen, in a form that cannot cause the disease.It…
- artificial active immunity
- Antigenic variation
- Many strains
- Hidden pathogens
- Pathogens living inside cells or having complex life cycles
- Individual differences
Section 1 of 9What a vaccine is and how it works
A vaccine contains one or more antigens from a pathogen, in a form that cannot cause the disease.
What a vaccine is and how it works
It may be a weakened (attenuated) live pathogen, a killed or inactivated pathogen, purified antigens (such as surface proteins), an inactivated toxin (toxoid), or genetic material (mRNA) that instructs the cell to make an antigen.
The vaccine is usually injected, although some are given by mouth.
Vaccination produces artificial active immunity:
1
The vaccine antigens are taken up by antigen-presenting cells and presented to T helper cells.
2
The T helper cells activate B cells with complementary receptors, which undergo clonal selection and expansion.
3
Plasma cells produce antibodies (the primary response, which is slow and low in this case), and B memory cells and T memory cells form.
4
If the real pathogen later enters the body, memory cells divide and differentiate rapidly (the secondary response), producing a high level of antibody quickly so that the pathogen is destroyed before symptoms develop.
Booster doses give a second stimulation and increase the number of memory cells.
Section 2 of 9Herd immunity
If a large enough proportion of a population is vaccinated, the pathogen cannot spread easily because most contacts of an infected person are immune.
This protects the people who are not vaccinated, such as babies, people with weak immune systems and those who cannot have the vaccine for medical reasons.
Herd immunity
This is called herd immunity.
The percentage needed depends on how infectious the disease is: for measles, which spreads very easily, about 95 percent coverage is needed, but for diseases with a lower transmission rate the figure is lower.
When vaccination rates fall, herd immunity is lost and outbreaks may occur. Eradication of smallpox (declared in 1980) was achieved by vaccination programmes, aided by the fact that smallpox has a single stable antigen, no animal reservoir and clear symptoms that made cases easy to identify.
Section 3 of 9Why vaccination is not always successful
1Antigenic variation
Antigenic variation pathogens such as influenza virus change their surface antigens by mutation, so memory cells no longer recognise them. New vaccines have to be produced for each season.
2Many strains
Many strains the common cold has many different types of virus, and a vaccine cannot cover them all.
3Hidden pathogens
Hidden pathogens some pathogens such as HIV hide inside cells, and the pathogen infects the immune cells themselves.
4Pathogens living inside cells or having complex life cycles
Pathogens living inside cells or having complex life cycles for example, the malaria parasite changes its antigens through its life cycle.
5Individual differences
Individual differences some people do not respond well because of poor health or age, or have side effects.
6Social and logistic reasons
Social and logistic reasons poor access to healthcare, a lack of cold storage in hot climates, cost, and public concerns about safety.
Section 4 of 9Vaccine types compared
| Type | Advantage | Disadvantage |
|---|---|---|
| Live attenuated | Strong, long-lasting response, often needing one or two doses | May cause disease in immunocompromised people; needs cold storage |
| Inactivated / killed | Cannot cause the disease | Weaker response, needs boosters |
| Toxoid or subunit | Safe, targeted antigens | May need adjuvants and boosters |
| mRNA | Rapid development, no live pathogen | Requires very cold storage in some cases |
Section 5 of 9Side effects and evaluation
Vaccines are tested in clinical trials for safety and efficacy before being used, and are monitored afterwards.
1
Side effects are usually mild, such as pain at the injection site or a short fever.
2
Rare serious reactions occur and must be balanced against the risk from the disease itself.
3
The benefit to the population (herd immunity) means that vaccination programmes are generally judged to be worthwhile when the disease is dangerous, the vaccine effective and risks low.
4
Evaluate any such question by using data on the incidence of the disease before and after the programme.
Section 6 of 9Data and graph interpretation
Vaccination programmes are judged using data on the number of cases per year.
1
A typical graph shows cases falling steeply after the introduction of a vaccine, and rising again if coverage drops.
2
When describing such a graph, quote the figures at two points and state the overall trend, for example 'cases fell from 22 000 to 300 over ten years'.
3
To calculate a percentage decrease, divide the fall by the original value and multiply by 100. Remember that a correlation between the start of the programme and the fall in cases does not by itself prove the vaccine was the cause, but when many countries show the same pattern, and the mechanism is understood, the evidence is strong.
4
Also compare vaccinated and unvaccinated groups: the group with a higher rate of disease is the unvaccinated one, and the size of the difference gives the effectiveness of the vaccine.
Section 7 of 9Exam-style questions
Explain how vaccination protects a person against a future infection by a pathogen. [4]4 marks
11. The vaccine contains antigens which stimulate B cells with complementary receptors to divide and form plasma cells and memory cells.22. Memory cells remain in the blood.33. On infection with the pathogen, memory cells divide rapidly and produce plasma cells.44. Antibodies are produced faster and in larger quantity so the pathogen is destroyed before symptoms develop.Explain what is meant by herd immunity and why it is important. [3]3 marks
Exam-style questions1. A large proportion of the population is immune through vaccination.2. So the pathogen has fewer susceptible hosts and cannot spread easily.3. Unvaccinated individuals (e.g. babies, immunocompromised) are therefore protected.Suggest why a new influenza vaccine is needed every year. [2]2 marks
1. Influenza virus mutates so surface antigens change (antigenic variation).
2. Memory cells from the previous vaccine do not recognise the new antigens, so a new vaccine is needed.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Vaccine | Preparation containing antigens that stimulates immunity without causing the disease |
| Active immunity | Immunity from making your own antibodies after exposure to an antigen |
| Herd immunity | Protection of a population when enough people are immune to stop the spread |
| Attenuated | Weakened so that it does not cause disease |
| Booster | Further dose that increases the number of memory cells |
| Antigenic variation | Change in surface antigens of a pathogen |
| Toxoid | Inactivated toxin used as a vaccine |
Section 9 of 9Exam technique
1
Saying a vaccine contains antibodies - it contains antigens (a vaccine gives active, not passive, immunity).
2
Stating that the vaccine itself kills the pathogen.
3
Forgetting to mention memory cells and the secondary response.
4
Saying herd immunity means everyone must be vaccinated - only a high proportion is needed.
5
Confusing attenuated (weakened live) with inactivated (killed).
Marked practiceQuizzes for this lesson
Last updated: