Antigen
Antigens are "targeted" by antibodies. Each antibody is specifically produced by the immune system to match an antigen after cells in the immune system come into "contact" with it; this allows a precise identification or matching of the antigen and the initiation of a tailored response. The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation in a region of the antibody; because of this, many different antibodies are produced, each able to bind a different antigen while sharing the same basic structure. In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react and bind more than one antigen.
Antigenic drift
The immune system recognizes viruses when antigens on the surfaces of virus particles bind to immune receptors that are specific for these antigens. This is similar to a lock recognizing a key. After an infection, the body produces many more of these virus-specific immune receptors, which prevent re-infection by this particular strain of the virus and produce acquired immunity. Similarly, a vaccine against a virus works by teaching the immune system to recognize the antigens exhibited by this virus. However, viral genomes are constantly mutating, producing new forms of these antigens. If one of these new forms of an antigen is sufficiently different from the old antigen, it will no longer bind to the receptors of the body cells, and viruses with these new antigens can infect the body cell as it avoids the immunity to the original strain of the virus. When such a change occurs, people who have had the illness in the past are therefore not immune to the new strain of the virus (as the new strain of the virus has a different antigen which the body cell cannot recognise) and thus the vaccines against the original virus will be less effective against the illness. Two processes drive the antigens to change: antigenic drift and antigenic shift, antigenic drift being the more common. The rate of antigenic drift is dependent on two characteristics: the duration of the epidemic, and the strength of host immunity. A longer epidemic allows for selection pressure to continue over an extended period of time and stronger host immune responses increase selection pressure for development of novel antigens.