It is helpful to learn the basic mechanism of how vaccines work so that there is a clear understanding of the strengths and limitations of vaccines.

When a foreign agent enters the body, the immune system sends sentry cells to protect itself. If an unrecognized agent is identified, the body produces antibodies or cell-based immune responses. In the immune system, cells are conditioned to recognize these new foreign agents and remember the important response information (memory cells). It is the memory cells that provide the key to long-term vaccine-induced protection against these agents. Memory cells may keep their programming for many years. Natural infections and vaccines both produce protective reactions in the body.

Boosters are essential for most vaccines, because the sufficient conditioning of memory cells requires multiple presentations of the foreign agent. This anamnestic response is a learned programmed reactivity, and one-time exposure may sometimes not be enough to trigger memory. An important thing to remember is that the animal is not considered fully vaccinated until two weeks after all of the booster series is complete. It takes 10-14 days following the vaccine administration in order to complete a full response.

In very young puppies and kittens, the maternal antibodies that their mother passes on to them interfere with the response to the vaccine. To counter this, boosters are given until the maternal interference naturally degenerates. Because puppies and kittens lose the protection of the mother's transferred antibodies at different ages, the vaccines are given over a course of time that covers the timeframe from earliest to latest age of maternal protection loss.