During a food poisoning outbreak, scientists identified the farm where the food came from. The farmer had not vaccinated the farm animals against Salmonella bacteria. The food poisoning outbreak could have been prevented if the farm animals had been vaccinated. Explain how:
• the immune systems of animals respond to a vaccination
• the immune response in farm animals prevents an outbreak of food poisoning in humans.

**Animal's response to vaccination:** A vaccine contains a harmless version of the pathogen, such as a dead or inactive form of *Salmonella*. This version still has specific molecules on its surface called antigens. When injected into the animal, its immune system recognises these antigens as foreign. White blood cells called lymphocytes are stimulated to produce antibodies that are a specific shape to bind to the *Salmonella* antigens. Crucially, the immune system also produces memory cells, which remain in the animal's blood for a long time. **How this prevents human outbreaks:** If a vaccinated animal is later exposed to the live, harmful *Salmonella* bacteria, the memory cells recognise the antigens immediately. This triggers a secondary immune response, which is much faster and stronger than the first. A large number of the correct antibodies are produced very quickly. These antibodies help to destroy the *Salmonella* bacteria before they can multiply to dangerous levels and cause disease in the animal. Because the bacteria are killed off, they cannot contaminate the animal's products (like meat or eggs), and therefore the pathogen is not passed on to humans, preventing a food poisoning outbreak.