Why do vaccines build up immunity?

The correct answer is: "The body produces antibodies when it is exposed to the weakened or dead form of a pathogen".

Explanation:

The vaccine consists on exposing the human immune system to a pathogen in a weakened form or even dead, so that antibodies are generated to fight it. If, subsequently, that same body is exposed to the "real" pathogen it will have memorized which are the necessary antibodies to sucessfully defeat it, and therefore the individual employs those antibodies and will not suffer that illness.