Introduction

In immunization, the body's ability to fight off certain disease-causing bacteria or viruses is stimulated or enhanced. There are two types of immunization, active and passive. Immunization is a method doctors use to protect people against certain diseases caused by bacteria and viruses.

In active immunization, vaccines are used to help prevent infection by stimulating the body's natural defense mechanisms. Vaccines are preparations that contain either noninfectious fragments of bacteria or viruses or whole forms of these organisms that have been weakened such that they 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 bacterium or virus contained in the vaccine. These antibodies and other substances are then automatically produced whenever the person is exposed to that specific bacterium or virus. 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. 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 exposure is likely and the person does not have time to get or complete a vaccination series. An example of this is the use of gamma globulin (an antibody preparation) 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.

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

Some vaccines are given routinely--for example, the tetanus toxoid is given to adults preferably every 10 years. Several different vaccines are routinely given to children. Other vaccines are given mainly to specific groups of people--for instance, 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 instance, the rabies vaccine may be given to a person who has been bitten by a dog.