Active immunization involves the use of vaccines, and passive immunization uses serums. Active immunization is another term for vaccination. A vaccine contains substances that stimulate the body's immune system to produce antibodies against a particular infectious disease. These antibodies protect the person if he or she is exposed to the actual disease-causing organism. Vaccines contain substances that are powerful enough to trigger antibody production but that do not actually cause disease.
Many vaccines consist of disease-causing bacteria or viruses that have been killed.
Others consist of the live germs, but in a weakened form that does not cause the disease. Vaccines known as toxoids are made from poisons produced by disease-causing organisms. These poisons are chemically treated so that they provide immunity without causing disease. Still other vaccines are made from parts of disease-causing organisms. Another group consists of live organisms that resemble disease-causing ones.