The immune system recognizes viruses when antigens on the surfaces of virus particles bind to immune receptors that are specific for these antigens. These receptors can be antibodies in the bloodstream or similar proteins on the surfaces of immune-system cells. This recognition is quite precise, like a key recognizing a lock. After an infection or after vaccination, the body produces many more of these virus-specific immune receptors, which prevent re-infection by this particular strain of the virus; this is called acquired immunity. 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 antibodies or immune-cell receptors, allowing the mutant virus to infect people who were immune to the original strain of the virus because of prior infection or vaccination.