Human defence systems and vaccination
The body fights pathogens with two layers of defence: non-specific barriers that don't care which pathogen is present, and a specific immune response that targets each pathogen.
Non-specific defences (first line)
| Site | Defence | How it works |
|---|---|---|
| Skin | Physical barrier | Tough outer layer; produces antimicrobial substances |
| Nose | Hairs and mucus | Trap particles and pathogens |
| Trachea / bronchi | Mucus and cilia | Mucus traps; cilia waft towards the throat |
| Stomach | Hydrochloric acid | Kills most ingested pathogens |
Specific defences — white blood cells
The immune system is built around white blood cells. Three main jobs:
- Phagocytosis — phagocytes engulf and digest pathogens.
- Antibody production — lymphocytes produce antibodies that lock onto specific antigens on the pathogen surface, marking pathogens for destruction or clumping them together.
- Antitoxin production — lymphocytes produce antitoxins that neutralise toxins released by bacteria.
Each pathogen has its own unique antigens, so a different antibody must be made for each. This is why we can catch many different colds — each is a different antigen.
Memory and the secondary response
After an infection, memory lymphocytes remain. If the same pathogen returns, they produce the right antibodies in larger amounts and more quickly — usually fast enough to destroy it before symptoms appear. This is immunity.
Vaccination
A vaccine introduces a small amount of dead or inactive pathogen (or its antigens) into the body. White blood cells:
- Recognise the antigens as foreign
- Produce the matching antibodies
- Form memory lymphocytes
If the live pathogen later enters the body, the secondary response is rapid — preventing illness. This works for measles, mumps, rubella, polio, tetanus and many more.
Herd immunity: when most of a population is vaccinated, the pathogen cannot spread effectively, protecting the few who can't be vaccinated (e.g. immunocompromised people).
Pros and cons of vaccination
Pros:
- Has eradicated smallpox; nearly eradicated polio
- Reduces spread of disease (herd immunity)
- Reduces healthcare costs
Cons:
- Vaccines are not 100% effective
- Some people have side effects (rare, usually minor)
- Some pathogens mutate so fast (e.g. flu) that new vaccines are needed yearly
⚠Common mistakes— Common mistakes / exam traps
- "Vaccines kill the pathogen directly" — they don't; they trigger memory lymphocyte formation so the body can respond fast in future.
- "Antibodies and antibiotics are the same" — antibodies are made by the body; antibiotics are drugs.
- Saying immunity is permanent for all infections — for many it is, but for fast-mutating viruses like flu, antigens change so memory cells no longer recognise them.
Links
Connects to B3.1 (pathogens), B3.3 (drug treatment) and B6 (mutation in pathogens drives the need for new vaccines).
AI-generated · claude-opus-4-7 · v3-deep-biology