Vaccines are like a training course for the immune system. They prepare the body to fight disease without exposing it to disease symptoms.
When foreign invaders such as bacteria or viruses enter the body, immune cells called lymphocytes respond by producing antibodies, which are protein molecules. These antibodies fight the invader known as an antigen and protect against further infection. According to the Centers for Disease Control and Prevention (CDC), a healthy individual can produce millions of antibodies a day, fighting infection so efficiently that people never even know they were exposed to an antigen.
Unfortunately, the first time the body faces a particular invader, it can take several days to ramp up this antibody response. For really nasty antigens like the measles virus or whooping cough bacteria, a few days is too long. The infection can spread and kill the person before the immune system can fight back. [Can You Get the Flu Twice in One Season?]
That's where vaccines come in. According to the Children's Hospital of Philadelphia Vaccine Education Center, vaccines are made of dead or weakened antigens. They can't cause an infection, but the immune system still sees them as an enemy and produces antibodies in response. After the threat has passed, many of the antibodies will break down, but immune cells called memory cells remain in the body.
When the body encounters that antigen again, the memory cells produce antibodies fast and strike down the invader before it's too late.
Vaccines also work on a community level. Some people can't be vaccinated, either because they are too young, or because their immune systems are too weak, according to the CDC. But if everyone around them is vaccinated, unvaccinated people are protected by something called herd immunity. In other words, they're unlikely to even come in contact with the disease, so they probably won't get sick. When it comes to vaccines, sometimes it can pay to follow the crowd.
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Originally published on Live Science.