There are two types of immunization – active and passive.
In active immunization, vaccines are used to stimulate the body’s natural defense mechanisms.
The body’s immune system responds to a vaccine by producing appropriate immune cells and memory cells to 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 immune cells.
In passive immunization, antibodies or activated immune cells against a specific infectious organism are given directly to a person. These antibodies and immune cells are obtained from several sources.
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 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 rarely have side effects, but they do not work in everyone. Some vaccines are routinely given to children for example polio vaccine, DPT, BCG etc.
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 following possible exposure to a specific disease.
For example, the rabies vaccine may be given to a person who has been bitten by a dog.
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) also.