Active Immunization

Active immunization is the process of challenging an individual with a foreign substance called a vaccine for the purpose of inducing a protective immune response against a known pathogen. The active ingredient(s) included in a vaccine are the specific components being used to stimulate the protective immune response in the recipient. Antigens are foreign substances capable of triggering an immune response; therefore, it is accurate to refer to the active ingredients present in a vaccine as “vaccine antigens.” These immunizing agents are also sometimes collectively referred to as immunogens. Based on the general characteristics of the immunogens, vaccines are easily grouped into the following categories: (1) replication-competent viruses and bacteria capable of inducing a protective immune response without causing disease, (2) whole, killed (inactivated) pathogens, and (3) a variety of viral and bacterial products that are either purified from native cultures or manufactured using recombinant technology.