Body & HealthHow it works
How your immune system learns a disease without having it.
A vaccine shows your body a harmless feature of a pathogen, such as a surface protein. The B and T cells that happen to match multiply over about two weeks, and some of them persist for years as memory cells. When the real pathogen arrives, the defence responds within days instead of a week, and with far more antibodies.
Useful: The same mechanism explains why you catch many childhood diseases only once.
Source: en.wikipedia.org