Antigenic Drift

Pathogens such as influenza virus have receptors that enable them to bind host cell surfaces, Antibodies to these viral receptors prevent the virus to binding to and infecting the cells, these are neutralizing antibodies since they neutralize the ability to infect the cell. However some virus have mutation that alter the receptor in the ways to prevent binding of neutralizing antibodies, while leaving the virus able to bind to and infect host cells.

In this way the pattern of antigens expressed by the virus can change over time. This process of accumulation of small changes is called has antigenic drift and contributes our susceptibility to influenza infections.