Preventing and treating disease

3. Infection and Response

The body defends itself in two ways: physical and chemical barriers that stop pathogens getting in at all, and the immune system, which destroys the ones that get through.

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What preventing and treating disease covers

Every subtopic below has its own questions, mark schemes and notes.

4.3.1.6Human defence systems

The body defends itself in two ways: physical and chemical barriers that stop pathogens getting in at all, and the immune system, which destroys the ones that get through.

4.3.1.7Vaccination

Vaccination gives the immune system a safe rehearsal, so that if the real pathogen ever arrives, the body can destroy it before it makes you ill.

4.3.1.8Antibiotics and painkillers

Some medicines only make you feel better, while antibiotics actually kill the bacteria causing the illness. Antibiotics do nothing against viruses, and overusing them is breeding bacteria they cannot kill.

4.3.1.9Discovery and development of drugs

Many medicines started life in a plant or a mould. Before any new drug reaches patients it has to survive a long testing process designed to show that it works, that it is safe, and at what dose.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Antigen — A unique molecule on the surface of a pathogen that the immune system recognises.

Antibody — A protein made by white blood cells that locks on to one specific antigen.

Antitoxin — A substance made by white blood cells that counteracts a toxin released by bacteria.

Phagocytosis — The engulfing and digesting of a pathogen by a white blood cell.

Cilia — Hair-like structures on the cells lining the airways that sweep mucus up towards the throat.

Immune — Able to destroy a particular pathogen quickly, so that it does not cause illness.

Vaccination — Introducing a small amount of a dead or inactive pathogen to make the body produce antibodies against it.

Inactive pathogen — A pathogen that has been weakened or killed so it cannot cause disease but still carries its antigens.

Epidemic — A large outbreak in which many people in an area catch a disease at the same time.

Immunity — The ability to destroy a particular pathogen quickly enough that it does not cause illness.

Antibiotic — A drug that kills bacteria inside the body, or stops them growing, without damaging body cells.

Painkiller — A drug that reduces the symptoms of an illness without killing the pathogen causing it.

Resistant — Describes bacteria that are not killed by a particular antibiotic.

Mutation — A random change in a gene, which can by chance make a bacterium resistant to an antibiotic.

MRSA — A strain of bacteria resistant to the antibiotic meticillin, known for causing serious wound infections.

Efficacy — Whether a drug works and produces the effect it is supposed to.

Toxicity — How harmful a drug is.

Dosage — How much of a drug should be given, and how often.

Placebo — A substance made to look like the drug being tested but with no active effect.

Double-blind trial — A trial in which neither the patient nor the doctor knows who is receiving the drug and who is receiving the placebo.

Optimum dose — The dose that is most effective while causing the fewest side effects.

Peer review — Checking of a scientist's work by other scientists before it is published.

Where marks get lost

The mistakes that come up again and again in this topic.

Antibodies and antitoxins are different things. Antibodies target the pathogen itself; antitoxins neutralise the poisons a bacterium releases.
Do not say white blood cells 'eat' or 'absorb' pathogens. The marking word is engulf, and the process is phagocytosis.
Antigens are on the pathogen; antibodies are made by you. Mixing the two up loses the mark even when the rest of the answer is right.
The stomach kills pathogens with acid, not with enzymes or mucus.
If a question asks why immunity to measles does not protect you from flu, the answer must mention that the antigens are different, not just that they are different diseases.
A vaccine contains a dead or inactive pathogen, not an antibody. The person makes their own antibodies; that is the whole point.
Say that white blood cells produce antibodies 'more quickly and in larger amounts' on the second meeting. 'They remember it' on its own is too vague for the mark.
A vaccine prevents a disease; it does not cure someone who already has it.
When explaining why unvaccinated people are protected, the mark is for there being fewer people who can pass the pathogen on, not simply that 'lots of people are immune'.
'Vaccines are dangerous' will not earn a mark. Bad reactions are described as rare, and an answer should say so.
Antibiotics kill bacteria only. Writing that they 'kill pathogens' or 'kill germs' will not get the mark, because it includes viruses.
Antibiotics do not make you immune and they are not vaccines.
In a resistance answer, the antibiotic does not cause the mutation. The mutation happens anyway; the antibiotic just kills everything that does not have it.
Say that resistant bacteria 'survive and reproduce', not that they 'become stronger' or 'learn to fight the antibiotic'.
A painkiller relieving symptoms is not the same as curing the disease, and examiners test exactly that distinction.
Preclinical testing means cells and tissues first, then live animals. Putting animals before cells loses the mark.
Clinical trials are on humans. If a question asks what a drug is tested on in a clinical trial, the answer is human volunteers, not cells or animals.
Healthy volunteers come before patients with the illness, and the reason is to check for side effects, not to see whether it works.
A placebo is not 'a fake pill given to trick people'. Say it has no active effect and is used so that the difference the drug makes can be measured.
Double-blind is about the doctor as well as the patient. An answer that only mentions the patient describes a blind trial, not a double-blind one.
Peer review needs both halves: other scientists check the work, and it helps prevent false claims being published.

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