AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Infection and response › Communicable diseases
Practise Human defence systems. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How your body stops pathogens getting in (the non-specific defences of the skin, nose, trachea and bronchi, and stomach) and how white blood cells destroy pathogens that do get in. A favourite for 'describe' and 'explain' questions, often in unfamiliar contexts such as smoking.
Key facts
Skin: physical barrier + antimicrobial secretions; scabs seal cuts
Nose: hairs and mucus trap particles
Trachea and bronchi: mucus traps pathogens; cilia move the mucus to the throat
Stomach: hydrochloric acid kills pathogens
White blood cells: phagocytosis, antibodies, antitoxins
Phagocytosis = engulfing and digesting a pathogen
Antigen = molecule (e.g. a protein) on the surface of a pathogen
Antibody: binds to a specific antigen on a pathogen
Antitoxin: neutralises a toxin made by bacteria
Notes
Non-specific defences: keeping pathogens out
diagram
Skin: a physical barrier that pathogens cannot get through while it is unbroken. It also produces antimicrobial secretions that kill pathogens. If the skin is cut, the blood clots and a scab seals the wound.
Nose: hairs and sticky mucus trap particles, which may contain pathogens, from the air you breathe in.
Trachea and bronchi: cells lining them make mucus, which traps pathogens. Other cells have cilia (tiny hairs) that beat to move the mucus up to the throat, where it is swallowed.
Mucus traps pathogens; cilia sweep it up to the throat.
Stomach: produces hydrochloric acid, which kills pathogens in food, drink and swallowed mucus.
These defences are non-specific: they work against all pathogens.
The immune system: white blood cells
diagram
If a pathogen gets into the body, the immune system tries to destroy it. White blood cells do this in three ways.
Phagocytosis: white blood cells called phagocytes engulf (surround) pathogens and digest them.
Phagocytosis: engulf, then digest.
Antibody production: white blood cells called lymphocytes make antibodies. Antibodies bind to antigens (molecules such as proteins) on the surface of a pathogen. This helps to destroy the pathogens, e.g. by sticking them together so phagocytes can engulf them.
Antibodies are specific: each one fits the antigen of only one type of pathogen, so a different antibody is needed for each pathogen.
Antibodies are specific: the shape must fit the antigen.
Antitoxin production: white blood cells make antitoxins, which bind to the toxins made by bacteria and neutralise them.
Why you can still get ill
The first time a pathogen gets in, it takes time for white blood cells to make the right antibodies.
Meanwhile the pathogen reproduces, so you may feel ill before it is destroyed.
How to answer each type of question
Describe how a part of the body stops pathogens
2 marksGrade 4
Name what does the job (mucus, cilia, acid, skin).
Say what it does to the pathogens (traps, moves away, kills, keeps out).
Example. Describe how the trachea and bronchi protect the body against pathogens.
Show the model answerHide the model answer
Mucus traps pathogens (1). Cilia beat to move the mucus up to the throat, where it is swallowed (1).
Describe how white blood cells defend the body
3 marksGrade 5
Give three separate ways: phagocytosis, antibodies, antitoxins.
Add what each one does, not just its name.
Example. Describe three ways that white blood cells help to defend the body against pathogens.
Show the model answerHide the model answer
They engulf and digest pathogens (phagocytosis) (1). They produce antibodies, which bind to antigens on the pathogens (1). They produce antitoxins, which neutralise the toxins made by bacteria (1).
Don’t lose marks
Mixing up antigen, antibody and antibiotic. Antigen = on the pathogen; antibody = made by white blood cells; antibiotic = a medicine that kills bacteria.
Saying white blood cells 'eat' pathogens without saying that they engulf and digest them.