5.12–5.13Human defence systems
The body's first line is non-specific — it works against anything, rather than against one pathogen. Four defences are named, and each has a mechanism worth stating rather than just naming.
If a pathogen gets past all of that and enters the body, the immune system tries to destroy it. White blood cells defend against pathogens in three ways: phagocytosis, antibody production and antitoxin production.
In phagocytosis the white blood cell engulfs the pathogen and digests it. Antibodies are proteins that lock onto the antigens on a particular pathogen's surface, so they are specific — an antibody for measles does nothing against gonorrhoea. Antitoxins neutralise the toxins that bacteria produce.
Common mistake
Antibodies and antitoxins are not the same, and neither is an antibiotic. Antibodies lock onto the pathogen, antitoxins neutralise its poisons, and antibiotics are a medicine you take. All three appear in the same questions.
- Pathogen
- a microorganism that causes infectious disease
- Antibody
- a protein produced by white blood cells that binds to a specific pathogen so it can be destroyed
- Antitoxin
- a substance produced by white blood cells that neutralises the toxins released by pathogens
5.14–5.15BVaccination
Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body, to stimulate the white blood cells to produce antibodies. If the same pathogen re-enters the body, the white blood cells respond quickly to produce the correct antibodies, preventing infection.
Two things to explain, and questions usually want both: how a vaccine protects the individual, and how immunising a large proportion of the population reduces the spread of a pathogen. The second is the one students forget — if most people cannot catch it, the pathogen runs out of hosts to move between, which protects even those who are not vaccinated.
You may also be asked to evaluate the global use of vaccination in the prevention of disease, which means weighing both sides and reaching a judgement. In favour: vaccination has eradicated smallpox worldwide and made diseases such as measles and polio rare where uptake is high; it prevents epidemics when enough of a population is immune; and it is far cheaper than treating the disease. Against, or at least complicating it: no vaccine works in everyone, some people cannot be vaccinated for medical reasons, side effects occur in a small number of cases, and cold storage and delivery make global coverage hard and expensive. Falling uptake in one community can bring a disease back for everyone.
You do not need details of vaccination schedules or the side effects of specific vaccines.
- Vaccination
- introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate white blood cells to produce antibodies, so that a later infection is met quickly
- Herd immunity
- the protection of a population when enough of it is vaccinated that a pathogen cannot spread readily
Before you practise
- Non-specific defences first: skin, nose hairs and mucus, the cilia of the trachea and bronchi, and stomach acid.
- White blood cells do three things: engulf pathogens, produce antibodies, and produce antitoxins.
- An antibody is specific to one antigen, which is why immunity to one disease gives none against another.
- A vaccine contains a small quantity of dead or inactive pathogen, which triggers antibody production without causing the disease.
- The second exposure produces antibodies faster and in greater quantity, so the pathogen is destroyed before symptoms appear.
- Herd immunity: if enough people are vaccinated the pathogen cannot spread, which protects those who are not.