High Quality Content by WIKIPEDIA articles! 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 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 and viruses with these new antigens can evade immunity to the original strain of the virus. When such a changes occurs, people who have had the illness in the past will lose their immunity to the new strain and vaccines against the original virus will also become less effective. Two processes drive the antigens to change: antigenic drift and antigenic shift, antigenic drift being the more common.

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