1.   A vaccine prevents viruses or bacteria from lodging and reproducing in the body and triggers cells of the immune system that neutralize or destroy the invading foreign organisms.

2.   Effective immune systems must respond quickly to invading organisms, such as viruses or bacteria, while not attacking normal tissues of the host organism.

3.   Ordinarily, these pea-size glands are filled with cells called lymphocytes, which produce the antibodies needed to fight off invading organisms such as bacteria or viruses.

4.   Vaccines turn on the immune system to stimulate antibodies that can kill or weaken the invading organism, in the process creating immunity to the disease.