On 11 December 1945, Alexander Fleming gave his Nobel lecture on penicillin, the prize he shared with Howard Florey and Ernst Chain. Penicillin was being hailed as a wonder drug, yet near the end of his talk Fleming gave a warning. Bacteria exposed to too little penicillin in the laboratory could easily be made resistant to it, he said, and this had occasionally happened in the body too. He worried that people taking too small a dose would breed resistant bacteria inside themselves. Resistant bacteria are now one of the biggest concerns in medicine, and AQA asks you to explain why.
This lesson covers the immune system, vaccination, antibiotics and drug development in AQA GCSE Biology Topic 3 (Infection and response). It's also useful for Combined Science, apart from the monoclonal antibodies section.
Keeping pathogens out
Your first defences are non-specific: they work against any pathogen.
- Skin is a tough barrier.
- The nose traps particles in hairs and mucus.
- In the trachea and bronchi, mucus traps pathogens and cilia sweep it up to the throat.
- The stomach makes hydrochloric acid, which kills most pathogens in food and drink.
The immune system
If a pathogen gets in, the immune system tries to destroy it. White blood cells do this in three ways:
- Phagocytosis: engulfing and digesting the pathogen.
- Antibody production: antibodies fit the antigens on one particular pathogen.
- Antitoxin production: antitoxins neutralise the toxins bacteria release.
The first time you meet a pathogen, making enough of the right antibody takes time, so the pathogen multiplies and you feel ill.
Vaccination
A vaccine contains small quantities of a dead or inactive form of a pathogen. It stimulates your white blood cells to make the right antibodies. If the live pathogen enters later, they respond quickly and stop the infection before you become ill.
When a large proportion of a population is vaccinated, the pathogen has few people to spread to. This herd immunity also protects people who aren't vaccinated. A worldwide vaccination campaign led to smallpox being declared eradicated by the World Health Organization in 1980. You don't need to know vaccination schedules or side effects of specific vaccines.
Antibiotics and painkillers
Antibiotics, such as penicillin, kill infective bacteria inside the body. Specific bacteria need specific antibiotics. They have greatly reduced deaths from bacterial disease, but resistant strains, such as MRSA, are a serious problem (you meet how resistance evolves in Topic 6).
Antibiotics cannot kill viruses. Drugs that kill viruses are hard to develop, because viruses live inside body cells and it's difficult to destroy them without damaging the body's tissues. Painkillers treat symptoms only; they don't kill the pathogen.
Discovering and testing drugs
Traditionally, drugs came from plants and microorganisms: digitalis from foxgloves (the physician William Withering published his account of foxglove as a medicine in 1785), aspirin from willow, and penicillin from the Penicillium mould. Most new drugs are now made by chemists, though the starting point may still be a plant chemical.
New drugs are tested for toxicity, efficacy and dose:
- Preclinical testing in a laboratory on cells, tissues and live animals.
- Clinical trials on healthy volunteers and patients. Very low doses come first; if the drug is safe, more trials find the optimum dose.
- In double-blind trials, some patients get a placebo, and neither they nor the doctors know who until the end.
Results are published only after peer review.
Monoclonal antibodies (Biology only, Higher tier)
Mouse lymphocytes are stimulated to make one antibody, then fused with a tumour cell to make a hybridoma cell, which can both divide and make the antibody. Cloning one hybridoma gives many identical cells making identical antibodies, specific to one antigen. They're used in pregnancy tests, to measure hormones or detect pathogens, to find molecules in research, and to deliver drugs to cancer cells. They have caused more side effects than expected.
Common exam mistakes
- "Vaccines contain antibodies." They contain dead or inactive pathogen; your white blood cells make the antibodies.
- "Antibiotics kill viruses." They only kill bacteria.
- "The body becomes resistant to antibiotics." The bacteria become resistant, not the person.
- "Painkillers cure the disease." They relieve symptoms only.
- Mixing up toxicity and efficacy. Toxicity is harm; efficacy is whether it works.
Frequently asked questions
How do vaccines work at GCSE?
A vaccine contains a dead or inactive pathogen, so white blood cells make the right antibodies. If the real pathogen enters later, they respond quickly and you don't become ill.
Why don't antibiotics work on viruses?
Antibiotics kill bacteria but can't kill viral pathogens, which reproduce inside your own cells.
What is herd immunity?
When most of a population is vaccinated, a pathogen can't spread easily, which protects the people who aren't vaccinated too.
What are the stages of drug testing at GCSE?
Preclinical testing on cells, tissues and live animals, then clinical trials on healthy volunteers and patients, starting with very low doses.
Try it
Say each answer before you flip the card. Then put yourself in Fleming's place in 1945: you know too little penicillin can make bacteria resistant in the lab, and you fear careless use will spread resistance in patients. What would you have tested next?
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