Immunization enables the body to better defend itself against diseases caused by certain bacteria or viruses. Immunization may occur on its own (when people are exposed to bacteria or viruses), or doctors may provide it. When people are immunized against a disease, they do not get the disease or get only a mild form of it.

There are two types of immunization: active and passive.

In active immunization, vaccines are used to stimulate the body's natural defense mechanisms. Vaccines are preparations that contain one of the following:

• Noninfectious fragments of bacteria or viruses
• A usually harmful substance (toxin) that is produced by a bacteria but has been modified to be harmless—called a toxoid
• Weakened (attenuated), live whole organisms that do not cause infection

The body's immune system responds to a vaccine by producing substances (such as antibodies and white blood cells) that recognize and attack the specific bacteria or virus contained in the vaccine. Then whenever the person is exposed to the specific bacteria or virus, the body automatically produces these antibodies and other substances. The process of giving a vaccine is called vaccination, although many doctors use the more general term immunization.

In passive immunization, antibodies against a specific infectious organism are given directly to a person. These antibodies are obtained from several sources:

• The blood (serum) of animals (usually horses) that have been exposed to a particular organism or toxin and have developed immunity
• Blood collected from a large group of people—called pooled human immune globulin
• People known to have antibodies to a particular disease (that is, people who have been immunized or who are recovering from the disease)—called hyperimmune globulin—because these people have higher levels of antibodies in their blood
• Antibody-producing cells (usually taken from mice) grown in a laboratory

Passive immunization is used for people whose immune system does not respond adequately to an infection or for people who acquire an infection before they can be vaccinated (for example, after exposure to the rabies virus). Passive immunization can also be used to prevent disease when people are likely to be exposed and do not have time to get or complete a vaccination series. For example, a solution containing gamma globulin (a common type of antibody) is used to help prevent hepatitis in people who travel to certain parts of the world. Passive immunization lasts for only a few days or weeks, until the body eliminates the injected antibodies.

Did You Know... Some vaccines contain a weakened but living form of the virus that they protect against.

Vaccines and antibodies are usually given by injection into a muscle (intramuscularly) or under the skin (subcutaneously). Antibodies are sometimes injected into a vein (intravenously).

Vaccines available today are highly reliable, and most people tolerate them well. They rarely have side effects, but they do not work in everyone.

Some vaccines are given routinely—for example, the tetanus toxoid is given to adults, preferably every 10 years. Some vaccines are routinely given to children.

Other vaccines are usually given mainly to specific groups of people. For example, the yellow fever vaccine is given only to people traveling to certain parts of Africa and South America. Still other vaccines are given after possible exposure to a specific disease. For example, the rabies vaccine may be given to a person who has been bitten by a dog.

Last full review/revision July 2008 by Fred H. Rubin, MD