Vaccines trigger the body’s adaptive immune response.

In 1721, an outbreak of smallpox in Britain grew into an epidemic. This made Britain’s royal family extremely nervous – and desperate for some form of protection. They had heard of an early form of vaccination for the disease but wanted it tested before it was used on their children.

On August 9, 1721, skin and pus from smallpox patients were rubbed into small cuts made on the arms and legs of six convicts. Another convict received a sample of skin and pus up her nose. The result? After a day or two of experiencing smallpox symptoms, all the convicts recovered.

These experiments seemed to prove that immune reactions are triggered when the body detects molecules it’s never encountered before. Then, if those same molecules appear in the body again, the immune system is ready to act. 

The key message here is: Vaccines trigger the body’s adaptive immune response.

You probably already know that vaccines are vital lifesavers. But you may not have known that vaccination has preserved more human lives than almost any other service. As the smallpox story illustrates, doctors were using vaccines to help people fight disease even before they knew how vaccines worked. The actual science behind them, in fact, took a long time to discover.

Before the 1980s, scientists knew this much: two types of white blood cells, T cells and B cells, lie at the heart of immune response. On their surfaces, these cells have receptors made of long, elaborate strings of proteins that can link up with matching proteins on other molecules. This allows the cells to work together to complete various tasks. 

So, if an immune cell’s receptors connect with something alien to your body, the immune cell gets “switched on” and then kills the germ or infected cell. The immune cell also multiplies, which allows your body to “remember” germs that have been inside it before and deal with them easily. This is the process that vaccines activate, and it’s known as your adaptive immune response.

Done and dusted, right? Well, not quite. If your body had an immune response every time a new substance entered it, you’d get sick every time you ate a new food. One scientist, Charles Janeway, knew there had to be more to the story.