How does the adaptive immune system have memory? Why is this helpful for staying healthy?

The adaptive immunity of a person's body has the ability to make different antibodies against different antigens. These memory cells of the adaptive immune cells remember the antigen which once invaded the body of a person. When next time, the same antigen attacks the body, the memory cells immediately recognize it and immediately a response is generated by making the particular type of antibodies.

This phenomenon is helpful because it allows response to be generated by the immune cells at a much faster rate once the body has recognized the antigen. It prevents a person from getting sick from the same antigen each time.