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Antigenic drift refers to slow, continuous and random changes of immunity-forming surface structures (antigens) of pathogenic microorganisms.
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An example of this is influenza viruses in which, for example, point mutations occur in the genes that code for haemagglutinin and another surface protein, neuraminidase.
Such mutations, which occur every 2-3 years, mainly alter the protein segments that are important for the binding of neutralizing antibodies. These are only insufficiently effective. Other mutations affect epitopes that are recognized by T cells and in particular by CD8 T cells.
This antigenic drift of the virus leads to the selection of antigenic variants. These can now be attacked less efficiently by the host's immune defenses. The result is a new infection, despite previous illness or vaccination, which is relatively mild (there is usually still a certain cross-reaction with antibodies and T cells that were produced against the earlier variant of the virus) because most people have a certain residual immunity.