How Do Vaccines Work to Prevent Illness?

The human immune system has an army of white blood cells (WBCs) on patrol. Their job is to recognize and eliminate potentially dangerous microbes that don't belong in the body, such as bacteria and viruses (pathogens). The following explains how vaccines work to to prime white blood cells to recognize a pathogen before infection even takes place. The Hib vaccine is used as an example.

Immune System's B-cells

To understand how vaccines work, it is first necessary to learn a little about the white blood cells that protect the body from infection. Certain types of WBCs, called a B-cells, are specialists. These white blood cells are like special agents that eventually get assigned to target a specific invading pathogen. Before a B-cell can be assigned to deal with that pathogen, it must first encounter it.

Once a B-cell meets the invader, it specializes even further. Some B-cells, called Plasma cells, create a recipe for antibodies. Antibodies are proteins that help the body more quickly and effectively eliminate an invading microbe. You can think of antibodies as weapons that help the immune system destroy the bad guys. Other B-cells, called Memory cells, remember that antibody recipe, so that if the invader shows up in the body again, antibodies against that invader can be quickly made.

How Vaccines Help the Immune System

When a B-cell first meets an invading bacterium or virus, it takes a couple of weeks for it to create the recipe for the antibodies that will help control the infection. Two weeks is a long time, and during that time, some pathogens can multiply to a point that the person (host) becomes very sick while waiting for the immune system to kick in and help with recovery.

A vaccine allows the WBCs to recognize the infectious agent ahead of time, before an actual infection occurs. This is possible because vaccines contain either weakened, killed or portions of a specific bacterium or virus. When B-cells encounter the vaccine, they are stimulated to develop a recipe for antibodies. Having this recipe already prepared allows B-cells to make antibodies very quickly if they ever encounter the real pathogen .

An Example: The Hib Vaccine

Haemophilus influenzae, is a Gram-negative bacterium that, before development of the HIb vaccine, was the most common cause of meningitis in children under two years of age. Prior to development of a vaccine, even with treatment, the mortality rate from this type of bacterial meningitis was about 2%, and of those that survived, 20 - 30% suffered long-term neurological damage, such as deafness and mental retardation.

In the 1990's, the Hib vaccine was developed. It contained parts of a specific type of Haemophilus influenzae capsule. The capsule is a gooey type of shell around an individual bacterium that interferes with the human immune system recognizing it, kind of like Harry Potter's invisibility cloak, but for bacteria.

Prior to development of the Hib vaccine, there were between 40 and 100 annual cases of Haemophilus bacterial meningitis per 100,000 people; once the vaccine became widely used, that number plummeted to 1.3 people per 100,000.

Sources

Bauman, R. (2004) Microbiology. Pearson Benjamin Cummings.
Schauer, C. (2009). Microbiologist at Kalamazoo Community College.