Pathogens, defences and vaccination
How disease spreads, how the body fights it, and how vaccines work.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 10.1.1Define pathogen and transmissible disease, and describe how pathogens are transmitted.
- 10.1.2Describe the body's defences and explain how vaccination produces immunity.
6 minute read
Pathogens, defences and vaccination
A pathogen is a disease-causing organism. A transmissible disease is one in which the pathogen can be passed from one host to another.
Transmission
- Direct contact — blood or other body fluids.
- Indirect — contaminated food or water, airborne droplets, or an animal vector such as a mosquito.
The body's defences
Barriers that stop entry:
- Skin — a physical barrier.
- Hairs and mucus in the nose trap pathogens; ciliated cells in the airways sweep mucus away.
- Stomach acid kills most pathogens swallowed in food.
- Clotting at a wound seals the entry point quickly.
White blood cells, if entry succeeds:
- Phagocytes engulf and digest pathogens — a general, non-specific defence.
- Lymphocytes produce antibodies, proteins with a shape specific to the antigens on one particular pathogen. Antibodies cause pathogens to clump together and mark them for destruction.
How vaccination works
This chain is worth learning exactly, because it is asked frequently:
- A weakened or dead pathogen, or its antigens, is put into the body.
- It cannot cause disease, but it carries the same antigens.
- Lymphocytes recognise the antigens and produce specific antibodies.
- Memory cells remain in the blood afterwards.
- If the real pathogen infects later, memory cells produce antibodies faster and in greater quantity, destroying it before symptoms develop.
Herd immunity
If enough of a population is vaccinated, the pathogen cannot spread easily, protecting even those who are not vaccinated.
Think of it like this
A vaccine is a fire drill. Nothing is actually burning, but the building practises the response — so when a real fire starts, everyone reacts immediately instead of working out what to do.
Worked examples
Method, step by step
Explain why a person vaccinated against measles does not become ill when later exposed to the measles virus.
- 1The vaccine introduced measles antigens, so lymphocytes produced specific antibodies.
- 2Memory cells specific to measles remained in the blood afterwards.
- 3On real exposure, memory cells produce large amounts of the correct antibody very quickly.
- 4The virus is destroyed before it can multiply enough to cause symptoms.
Memory cells from vaccination recognise the measles antigens and produce specific antibodies much faster and in greater quantity, destroying the virus before symptoms develop.
Common misconceptions
- Thinking a vaccine contains antibodies. It contains **antigens** (weakened/dead pathogen); your own lymphocytes make the antibodies.
- Confusing antigen and antibody. The antigen is on the pathogen; the antibody is the protein your body makes to match it.
- Believing antibodies are general-purpose. Each antibody is specific to one antigen shape, which is why one vaccine does not protect against other diseases.
In the exam
- For "explain how vaccination gives long-term immunity", the essential marking points are: antigens introduced → lymphocytes make specific antibodies → memory cells remain → faster, larger response on re-infection.
- Do not write that vaccines "kill the disease". They prepare the immune system; the immune system does the killing.