Topic 10.1 · Core
Body Defences Against Pathogens
For this syllabus point, Cambridge limits the required defences to five: skin, hairs in the nose, mucus, stomach acid, and white blood cells. The useful way to hold all five in your head at once is that the first four stop a pathogen from ever getting properly inside, and the fifth is what deals with it once it has.
Skin
The skin forms a physical barrier between the internal body and everything outside it. While it’s intact, it makes it genuinely difficult for pathogens to reach the tissue underneath. The exam-relevant idea here is narrow but important: skin’s main job is preventing entry, not fighting an infection that’s already established — so an answer that describes skin “killing germs” is describing the wrong mechanism.
Hairs in the nose
Hairs inside the nose trap particles and pathogens carried in the air you breathe in, before they get the chance to travel further down into the breathing system. It’s a filtering role, not a killing one.
Mucus
Mucus works on the same principle — it physically traps pathogens and particles so they can’t keep moving through the airway. The more detailed story of mucus and the ciliated cells that sweep it along belongs to gas exchange, not to this chapter; here, the required idea is simply that trapping happens.
Stomach acid
Unlike skin, nasal hairs and mucus, stomach acid doesn’t just trap — it actively destroys. The stomach’s strongly acidic conditions kill many of the pathogens you swallow, which is a chemical defence rather than a physical one, and it’s worth being able to state that distinction explicitly if a question asks you to compare defences.
White blood cells
Everything above reduces how many pathogens ever get in. White blood cells are what happens if some get past that first line anyway, and Cambridge expects two mechanisms here, both at Core level: phagocytosis, where a white blood cell engulfs a pathogen directly, and antibody production, where white blood cells produce proteins that act against the pathogen. You don’t need the lymphocyte/phagocyte classification at this stage — that finer distinction, along with the full antigen-antibody mechanism, belongs to the Supplement-only active immunity and antibodies content. What matters here is the shape of the whole system: barriers first, white blood cells second.
The pattern examiners are actually checking for
Put together, the required logic is: physical and chemical barriers reduce how many pathogens enter the body, and white blood cells act against the pathogens that get past them. A common way to lose marks here is treating all five defences as interchangeable — describing stomach acid as if it merely traps, for instance, or describing skin as though it produces antibodies. Keep each mechanism attached to the right structure, and keep the two-stage story — barrier, then white blood cell — visible in your answer even when the question only asks about one part of it.