noun, plural: antigenic variations
Antigenic variation is one of the ways by which an infectious agent evades a host immune response. For example, a pathogenic bacterium could alter its surface proteins and carbohydrates so as to circumvent the immune response of the host.
Antigenic variation makes the pathogen evade detection of the immune cells by altering their surface antigens. This gives the pathogen the advantage of repeatedly infecting the same host (re-infection) and therefore prolongs its stay within the host. It also increases the chance of being transmitted to a new host.
One of the immune responses of the host is to detect the surface antigens of the pathogen. Once detected, the immune cells would target the pathogen and destroy it. Some immune cells, such as B cells, produce antibodies that could detect and incite destruction of pathogens that bear similar antigens in case of re-exposure. However, certain pathogens are capable of antigenic variation and therefore would render the antibodies ineffective against them.
Some of the mechanisms employed to enable antigenic variation are gene conversion, site-specific DNA inversions, and hypermutation.