Artificial immunity, or artificially acquired immunity refers to immune defences against pathogens which the body has not acquired naturally. These include vaccines and, less commonly, immunoglobulin injections of antibodies.
Immunity is a state achieved when the body's immune system is capable of immediately recognizing and eradicating a given pathogen, such as a viral or bacterial infection. In most cases, even without assistance, the immune system will eventually produce antibodies capable of binding to the pathogens in question, and will then destroy them, all using a sophisticated network of several types of white blood cells. However, there are many cases where the body is not capable of making an adequate response, at least not before serious and potentially life-threatening illness sets in. For this reason, we have learned to augment the body's natural immune responses through artificial immunity.
The most common type of artificial immunity is a vaccine. A vaccine is something resembling a harmful pathogen (i.e. a virus), which has in some way been rendered harmless. However, it mimics the actual pathogen to the extent that the immune system cannot tell the difference. Essentially, then, the immune system is given a "safe" copy of the virus to practice against and develop antibodies to target. Then, if an actual exposure to the pathogen occurs later on, it will already be capable of targeting and eradicating it. The first vaccine was actually another virus - in the 18th century, Edward Jenner realized that people exposed to the harmless bovine illness cowpox also became immune to cowpox's far deadlier relative, smallpox. Smallpox once killed millions; it is now the first disease to have been entirely eradicated in the wild through medical intervention.
Today, the science of vaccines is much more systematic. Vaccines typically take the form of dead copies of the virus, or live (known as "attenuated") copies of a virus which have been crippled so that they cannot infect cells and cause disease. In other cases, like the tetanus and diphtheria vaccine, the vaccine is actually derived from the toxin secreted by the bacteria, rather then the bacteria themselves. In such cases the immune system can be prompted to react the next time it discovers the toxin. Today, the list of vaccine-preventable diseases is extremely long, and includes diseases such as hepatitis B, polio, and cholera. An annual vaccine is also prepared against influenza (the so-called "flu shot"), but that disease mutates so quickly that even a regularly updated vaccine cannot guarantee full protection.