Antigens can be bound by immune cell receptors in order to trigger an immune response. Binding and recognition mechanisms are used to detect threats to the body, including viruses, bacterium, and chemicals. Cells within the body display antigens which enable them to be identified from foreign substances. Immune system receptors bind to foreign antigens which trigger immune responses to eliminate the threat, once infection is resolved the body can produce more of those specific receptors to detect these specific antigens upon reinfection, functioning as an acquired immune response to prevent infections of the same antigen in the future.
Vaccinations work with this same acquired immune response by stimulating production of receptors without having to face the primary infection via introduction of non-threatening antigens into the body.
Vaccinations and acquired immune responses only function if the antigenic structure remains the same, when the structure become altered receptors will no longer bind and the acquired immune system will not detect the threat, such is the case with antigenic drift and shift. A patient which has suffered a specific viral infection can become ill a second time, altering the structure of the antigen surface provides a potential mechanism for evasion of immune responses by pathogens.
Surface antigens of influenza include neuraminidase which functions in the process of new virions budding from cells, and hemagglutinin which bind to epithelial cells and allow entry of the virion.
As influenza replicates RNA to reproduce errors can be made that lead to mutations. 1 in 10000 nucleotides are transcribed incorrectly, which can be translated to 1 mutation ever time influenza replicates. RNA lacks ability to correct mutations that become permanent changes genetic code, each time influenza replicates progeny are slightly different. Differences accumulate with time and produce antigens that are substantially different from that of their ancestors, their antigens drift meaning that they can no longer be detected by acquired immune systems, and patients who have had one strain can be reinfected as the body will not detect it as the same infection. Antigenic drift is a serious and major concern for vaccinations, as slight changes in antigen structure mean previous vaccines will not be viable, requiring new vaccines.
