An antibody is a special protein your body makes to fight off germs and diseases. When a virus, bacterium, or other invader enters your body, your immune system creates antibodies specifically designed to recognize and attack that particular intruder, like a lock that only fits one key.

Think of antibodies as your body's wanted posters. Once your immune system encounters a virus, it creates antibodies that “remember” exactly what that virus looks like. If the same virus ever comes back, those antibodies spring into action quickly, helping you fight it off before you get sick. This is why you usually only get chickenpox once: your body keeps the antibodies around, ready to defend you.

Vaccines work by training your body to make antibodies without you having to get seriously ill first. The vaccine introduces a weakened or inactive form of a germ, giving your immune system practice making the right antibodies. Then if you ever encounter the real disease, your body already knows how to fight it.

Scientists can also measure antibodies in your blood to tell whether you've had certain diseases or received certain vaccines. Each antibody is remarkably specific: the ones that fight off flu won't help against a cold, which is why you need different antibodies for different illnesses.