Active, passive, and delayed immunity are all variations on the operation of the immune system, whereby antibodies are produced in response to the presence of an antigen considered to be foreign.

Active immunity occurs due to the production of an antibody as a result of the presence of the target antigen either as part of an intact infecting organism, or because of the introduction of the specific antigen in the form of a vaccine. The immunity is provided by an individual's own immune system.

The type of immunity invoked by the active response tends to be permanent. Once the antibody has been produced, an individual will be protected against the presence of the target antigen for a lifetime. The immune system has a capacity for memory of the antigen. If presented with the antigen challenge again, the immune machinery responsible...