The circulatory system is comprised of both red blood cells and white blood cells. White blood cells are part of the immune system and continuously fight infection. White blood cells, or leukocytes, are produced in the bones. Their primary function is to identify and destroy harmful antigens that enter the body. This is called the immune response--the way the body recognizes and defends itself against bacteria, viruses, and substances recognized as foreign and potentially harmful to the body.
Once the white blood cells encounter an antigen, or foreign substance, they are stimulated to divide and produce specialized cells to kill the substance.
If the body becomes infected with bacteria, the white blood cells go through a trial-and-error period to determine the best way to kill the bacteria. When the white blood cells determine how to destroy the bacteria, they create "memory cells" to handle future attacks of that same bacteria.
Vaccination is a way to trigger the immune response. When a person receives a vaccination, a very small amount of a disease-causing agent, called a vaccine, is injected into the body for the immune system to identify and then destroy. The amount of the bacteria in the vaccine is very small and is often weakened or dead. It usually does not cause illness. The body is then able to destroy the bacteria in the vaccine. Receiving a vaccination activates the immune system's "memory," allowing the body to react quickly by releasing antibodies to future exposures and thereby destroying the bacteria before it can cause illness. When the body develops immunity to a specific antigen, immunization has occurred.