Immunization, or immunisation, is the process by which an individual's immune system becomes fortified against an agent (known as the immunogen).
When this system is exposed to molecules that are foreign to the body, called non-self, it will orchestrate an immune response, and it will also develop the ability to quickly respond to a subsequent encounter because of immunological memory. This is a function of the adaptive immune system. Therefore, by exposing an animal to an immunogen in a controlled way, its body can learn to protect itself: this is called active immunization.
The most important elements of the immune system that are improved by immunization are the T cells, B cells, and the antibodies B cells produce. Memory B cells and memory T cells are responsible for a swift response to a second encounter with a foreign molecule. Passive immunization is direct introduction of these elements into the body, instead of production of these elements by the body itself.
Immunization is done through various techniques, most commonly vaccination. Vaccines against microorganisms that cause diseases can prepare the body's immune system, thus helping to fight or prevent an infection. The fact that mutations can cause cancercells to produce proteins or other molecules that are known to the body forms the theoretical basis for therapeutic cancer vaccines. Other molecules can be used for immunization as well, for example in experimental vaccines against nicotine (NicVAX) or the hormone ghrelin in experiments to create an obesity vaccine.
Immunizations are often widely stated as less risky and an easier way to become immune to a particular disease than risking a milder form of the disease itself. They are important for both adults and children in that they can protect us from the many diseases out there. Immunization not only protects children against deadly diseases but also helps in developing children's immune systems. Through the use of immunizations, some infections and diseases have almost completely been eradicated throughout the United States and the World. One example is polio. Thanks to dedicated health care professionals and the parents of children who vaccinated on schedule, polio has been eliminated in the U.S. since 1979. Polio is still found in other parts of the world so certain people could still be at risk of getting it. This includes those people who have never had the vaccine, those who didn't receive all doses of the vaccine, or those traveling to areas of the world where polio is still prevalent.
Active immunization/vaccination has been named one of the "Ten Great Public Health Achievements in the 20th Century".
Before the introduction of vaccines, people could only become immune to an infectious disease by contracting the disease and surviving it. Smallpox (variola) was prevented in this way by inoculation, which produced a milder effect than the natural disease. The first clear reference to smallpox inoculation was made by the Chinese author Wan Quan (1499–1582) in his Douzhen xinfa (痘疹心法) published in 1549. In China, powdered smallpox scabs were blown up the noses of the healthy. The patients would then develop a mild case of the disease and from then on were immune to it. The technique did have a 0.5–2.0% mortality rate, but that was considerably less than the 20–30% mortality rate of the disease itself. Two reports on the Chinese practice of inoculation were received by the Royal Society in London in 1700; one by Dr. Martin Lister who received a report by an employee of the East India Company stationed in China and another by Clopton Havers. According to Voltaire (1742), the Turks derived their use of inoculation from neighbouring Circassia. Voltaire does not speculate on where the Circassians derived their technique from, though he reports that the Chinese have practiced it "these hundred years".It was introduced into England from Turkey by Lady Mary Wortley Montagu in 1721 and used by Zabdiel Boylston in Boston the same year. In 1798 Edward Jenner introduced inoculation with cowpox (smallpox vaccine), a much safer procedure. This procedure, referred to as vaccination, gradually replaced smallpox inoculation, now called variolation to distinguish it from vaccination. Until the 1880s vaccine/vaccination referred only to smallpox, but Louis Pasteur developed immunization methods for chicken cholera and anthrax in animals and for human rabies, and suggested that the terms vaccine/vaccination should be extended to cover the new procedures. This can cause confusion if care is not taken to specify which vaccine is used e.g. measles vaccine or influenza vaccine.