How Do Vaccines Work to Fight Infection

How do vaccines work to fight infection?

A) They increase the body's resistance to infections in general by strengthening the immune system.
B) They introduce the immune system to a particular antigen so it will produce memory cells ready to fight the infection if it strikes again.
C) They mildly infect the body so the immune system can practice fighting off a particular infection so it will do a better job with the real thing.
D) They prevent all subsequent infections from occurring.
E) They enhance the ability of the T cells to produce antibodies later on.