How Vaccines Train the Immune System

A The human body is under constant threat from a host of tiny invaders: the bacteria and viruses that cause disease. To defend itself it possesses an elaborate system of protection, known as the immune system, which is able to recognise these invaders, attack them and drive them out. Remarkably, this system does not merely fight off an infection and forget it. Once it has met a particular germ and overcome it, the immune system remembers that enemy, so that if the same germ ever returns it can be dealt with far more swiftly and before it has a chance to make the person ill. This lasting protection is what we call immunity.

B This ability to remember is the foundation of one of the greatest achievements in the history of medicine. The idea behind vaccination is beautifully simple: instead of waiting for a dangerous germ to strike, doctors deliberately show the immune system a harmless preview of that germ in advance. The body responds as though it were under real attack, building up its defences and, crucially, forming a lasting memory of the invader, yet without the person ever suffering the disease itself. When the real germ appears later, the body is already prepared and defeats it before it can take hold.

C A vaccine can present this harmless preview in several different forms. Some vaccines contain the germ itself, but weakened or killed so that it can no longer cause illness. Others contain only a single part of the germ, such as one of the proteins from its outer coat, which is enough for the immune system to recognise but cannot make anyone sick. More recent vaccines carry only a set of instructions that the body's own cells use to make that harmless fragment for a short time. In every case the aim is the same: to give the immune system enough of a taste of the enemy to learn its shape, while never exposing the body to real danger.

D When a vaccine is given, the immune system swings into action much as it would against a genuine infection. It produces defensive proteins called antibodies, which lock onto the invader and mark it for destruction, and it creates long-lived memory cells that can remain in the body for years, sometimes for a lifetime. These memory cells are the key to lasting protection. Should the real germ ever enter the body, they recognise it at once and launch a rapid, powerful response, producing fresh antibodies so quickly that the infection is stopped almost before it begins.

E The benefits of vaccination reach beyond the individual who receives it. When most of the people in a community are protected, a germ finds very few people it can infect, and so it struggles to spread from one person to the next. In this way even those who cannot be vaccinated, such as newborn babies or people who are seriously ill, gain a measure of protection, because the disease can no longer move easily through the population around them. It was this power, applied worldwide over many years, that made it possible to wipe out smallpox, once one of the most feared of all diseases, from the face of the Earth entirely.

F Vaccination is not without its challenges. New vaccines must be tested with great care to make sure they are both safe and effective, a process that can take years. Some germs change so quickly that a vaccine which works one year may be less effective the next, which is why the vaccine against influenza has to be updated regularly. And a vaccine can only protect a population if enough people actually receive it. Yet despite these difficulties, vaccination remains one of the most powerful and cost-effective tools that medicine has ever devised, having saved many millions of lives and freed whole generations from diseases that once killed or crippled without mercy.