Active immunization is the induction of immunity after exposure to an antigen

Antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

. Antibodies

Antibody

An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

are created by the recipient and may be stored permanently.

Active immunization can occur naturally when a microbe or other antigen is received by a person who has not yet come into contact with the microbe and has no pre-made antibodies for defense. The immune system will eventually create antibodies for the microbe, but this is a slow process and, if the microbe is deadly, there may not be enough time for the antibodies to begin being used.

Artificial active immunization is where the microbe is injected into the person before they are able to take it in naturally. The microbe is treated, so that it will not harm the infected person. Depending on the type of disease, this technique also works with dead microbes, parts of the microbe, or treated toxins from the microbe.