Body defences and control of spread
Five named body defences: skin (a physical barrier), hairs in the nose (trap larger particles), mucus (a sticky secretion that traps pathogens), stomach acid (kills pathogens swallowed with food or drink) and white blood cells (engulf pathogens, or produce antibodies). Five named population measures: a clean water supply, hygienic food preparation, good personal hygiene, waste disposal and sewage treatment. The detailed stages of sewage treatment are not required.
(Extended) Active immunity
Active immunity is defence against a pathogen by the body producing its own antibodies, gained after an infection or by vaccination. Each pathogen carries its own antigens, molecules of a specific shape. Antibodies are proteins whose shape is complementary to one antigen; binding leads either to direct destruction of the pathogen or to it being marked for destruction by phagocytes. Vaccination also leaves memory cells, which give long-term immunity.
Pathogen and transmissible disease
A pathogen is a disease-causing organism — a bacterium, virus, fungus or protozoan. A transmissible disease is one in which the pathogen can be passed from one host to another. Transmission is either direct, by contact between hosts, including through blood and other body fluids, or indirect, from contaminated surfaces, food, animals or air. Classifying a given route as direct or indirect is a routinely tested skill, so learn the two lists as lists.
Drawn from real examiner reports.
Antibiotic resistance is not immunity
Immunity belongs to the host: the ability to defend against a pathogen, gained by infection or by vaccination. Antibiotic resistance belongs to the pathogen: bacteria that survive exposure to a drug. Taking an antibiotic does not make a person immune, and resistant bacteria are not immune — they are simply not killed by that antibiotic.
November 2022 sitting, Paper 42 Q4a(ii): candidates confused antibiotic resistance with immunity.
Bacteria do not acquire resistance
Resistance is not picked up because the bacteria met the antibiotic. A few bacteria already carry the resistant allele before treatment. The antibiotic kills the non-resistant ones; the resistant ones survive, reproduce and pass the allele on, so the proportion of resistant bacteria rises over generations. Write it as survival of bacteria that were already resistant.
June 2023 sitting, Paper 41 Q7a: candidates described bacterial resistance as something bacteria "acquire" or "develop" during exposure, rather than as natural selection acting on existing genetic variation.
Vague control measures score nothing
Asked how the spread of a disease can be controlled, name a specific measure and, where asked, how it works. For a disease passed in blood or body fluids, credit goes to using a barrier method of contraception, or to sterilising and not sharing needles. Answers such as "safe sex" or "be careful" state nothing that could actually be done, so they earn no mark.
November 2022 sitting, Paper 42 Q1a(ii): candidates gave vague answers such as "safe sex" instead of the specific measures (barrier contraception, sterilising needles) the mark scheme required.
Mucus traps, cilia move it
Mucus is one of the five named body defences: a sticky secretion that traps pathogens and particles. Cilia are the tiny hair-like structures that beat to sweep that mucus, and everything caught in it, out of the airways. They work together but do different jobs, so give the role the question actually asks for instead of treating the two as interchangeable.
November 2023 sitting, Paper 41 Q1b(i): asked which structure produces the mucus, candidates named ciliated cells or cilia — cilia move mucus, they do not make it.
Antigen and antibody get swapped
An antigen is part of the pathogen: a molecule with a specific shape on its surface. An antibody is made by the host, by lymphocytes, and binds to a matching antigen. Pathogens carry antigens; hosts make antibodies. Swapping the two words reverses the whole mechanism, so a sentence such as "the pathogen releases antibodies" cannot be credited.
A vaccine does not contain antibodies
A vaccine puts weakened pathogens, or their antigens, into the body. The antibodies are then made by the person's own lymphocytes — that is exactly why this counts as active immunity and why memory cells remain afterwards. Writing that the vaccine injects ready-made antibodies skips the lymphocyte and memory-cell steps, which are where most of the marks sit.
Indirect routes get mislabelled direct
Direct transmission means the pathogen passes between two hosts by contact, including through blood and other body fluids. Everything else is indirect: a contaminated surface, contaminated food, an animal such as a mosquito, or the air. Droplets from a cough and pathogens in drinking water are indirect, even though an infected person is obviously the original source.
Count your points against the tariff
The vaccination chain has five steps: weakened antigen in, lymphocytes stimulated, antibodies produced, memory cells remain, faster response on re-infection. Give as many steps as there are marks. Stopping at "antibodies are produced" is the commonest way marks are lost.
Name the defence and its mechanism
"Describe" questions on body defences want the structure and what it does: mucus traps pathogens, stomach acid kills them, skin is a physical barrier. A bare list of five nouns rarely reaches half marks. One verb per defence turns a list into a description.
Answer at the level asked
Immunity questions come at two levels. Individual: antibodies and memory cells inside one person. Population — the role of vaccination in controlling spread — needs the extra step: an immune person cannot catch and pass the pathogen on, so fewer people remain able to transmit it.
Say complementary shapes out loud
On any antibody question, state the shape-matching idea: the antibody binds only that antigen because their shapes are complementary. Then give both outcomes: direct destruction, or marking for destruction by phagocytes. "Antibodies attack pathogens" is not enough.
(Supplement) Define an antibody, including how it binds to its target.
(a) Define a pathogen. (1 mark) (b) Define a transmissible disease. (1 mark) (c) State whether the following is an example of direct or indirect transmission: a person becomes infected after drinking water contaminated with a pathogen from untreated sewage. (1 mark)
