What does a vaccine do to help an immune system?

A vaccine works by training the immune system to recognize and combat pathogens, either viruses or bacteria. To do this, certain molecules from the pathogen must be introduced into the body to trigger an immune response.

Explanation:

Basically it trains the body by giving it a little expirementation withthe  sickness. After that, the body will learn how to properly fight or keep the sickness at bay.

Hope it helped!;)