Vaccines help acquaint the body's immune system with an infectious agent, such as a virus or bacteria, without actually causing the disease. To guarantee that the vaccine will work and will be safe after transport and storage, several ingredients go into the vaccine shot.

Immune System

The immune system is comprised of specialized cells and proteins that attack and destroy invading organisms. The immune system then "remembers" the organism in order to better fight it the next time. You are usually immune to a disease after your first bout with it.

Attenuated or Killed Organisms

Vaccines expose the immune system to a killed or attenuated infectious organism, or only a piece of the organism, without causing disease. The immune system reacts to it as if the organism was alive and causing disease.

Adjuvants

Adjuvants are chemicals added to vaccines to make the immune system react more strongly. These chemicals cut down on the amount of organism that must be included in the vaccine, thus making more vaccine available.

Preservatives

Vaccines have to be transported from manufacturing facilities to the site where they will be administered. Preservatives are added in order to prevent any possible bacterial contamination from growing in the vaccine.

Other Ingredients

The dead or attenuated viruses and bacteria in vaccines are grown on cell cultures or other media, like eggs. Though the manufacturing process eliminates most of the incidental ingredients, like cell proteins or egg proteins, some of these make it into the vaccines.

Source:

Centers for Disease Control and Prevention

Hartford University Course on Immunity

Food and Drug Administration