How is a vaccine able to produce immunity?

Answer 1

A vaccine is a substance that contains an anti-gen to which the immune system responds. Antigens can be derived from living but attenuated (weakened) organisms, dead organisms, or parts of organisms.
"When the vaccine or toxoid is administered, the immune system recognizes it as foreign and produces antibodies, or sometimes cytotoxic T cells, and memory cells.
This immune response is the same as the one that occurs during the course of a disease. The disease itself does not occur either because whole organisms are not used or because they have been sufficiently weakened to have lost their virulence. In other words, vaccines retain important antigenic properties but lack the ability to cause disease.