Immunity, vaccines and medicines

The body has layers of defence against pathogens. Barriers such as the skin stop most of them getting in; white blood cells destroy those that do. Vaccines train the immune system before we meet a disease, and medicines such as antibiotics help us recover. New drugs are tested carefully before anyone can use them.

Key facts

• Non-specific defences: the skin is a barrier; the nose traps particles with hairs and mucus; the trachea and bronchi are lined with mucus and cilia; the stomach makes hydrochloric acid that kills pathogens.
• White blood cells destroy pathogens by phagocytosis (engulfing them), by making antibodies, which lock onto the antigens of one kind of pathogen, and by making antitoxins, which neutralise toxins.
• Vaccination injects a small amount of a dead or inactive pathogen. White blood cells make antibodies and memory cells, so if the real pathogen enters, antibodies are made quickly and in large amounts. If most people are vaccinated, the disease cannot spread easily: herd immunity.
• Antibiotics, such as penicillin, kill bacteria inside the body; they do not work on viruses. Bacteria can become resistant. Painkillers ease symptoms but do not kill pathogens.
• Many drugs were first found in living things: digitalis from foxgloves, aspirin from willow, and penicillin from the Penicillium mould, discovered by Alexander Fleming.
• New drugs are tested for toxicity, efficacy and dose: first on cells, tissues and animals, then in low doses on healthy volunteers, then on patients. Double-blind trials with a placebo stop bias, and results are peer reviewed.
• Monoclonal antibodies are identical antibodies made from one clone of cells. They bind to one antigen and are used in pregnancy tests, to diagnose disease and to target cancer cells.

Worked examples