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!;)

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms