Antigenic variation

Antigenic variation is a pathogen’s alteration of surface molecules recognized by the immune system. It can help pathogens evade existing immune responses and persist or spread.

Gene conversion: A process that copies sequence information from one DNA region into another, altering a gene without reciprocal exchange. Some pathogens use it to replace expressed antigen genes with silent variants.
Influenza A virus: An influenza virus with a segmented RNA genome that infects humans and other animals. Its surface glycoproteins change through drift and, more rarely, reassortment-driven shift.
Antigen: A molecule or molecular structure that can be specifically recognized by antibodies or immune receptors. Variation matters when it changes the structures immune defenses can recognize.
Relapsing fever: A tick- or louse-borne illness marked by recurring episodes of fever, commonly caused by Borrelia bacteria. Antigen switching can let Borrelia repopulate the blood after each immune-driven decline.
Trypanosoma brucei: A protozoan parasite that causes African trypanosomiasis and changes its exposed variant surface glycoprotein. Its successive surface-coat switches are a classic mechanism of antigenic variation.
Neisseria gonorrhoeae: A bacterium that causes gonorrhea and can vary its pili and other surface structures. Its changing pilin helps frustrate antibody responses during infection.
Epitope: The specific part of an antigen bound by an antibody or recognized by an immune receptor. A surface change can alter or conceal the precise target of existing antibodies.
Antigenic drift: The gradual accumulation of mutations that alter viral antigens, especially in influenza viruses. Repeated small antigen changes can weaken recognition by antibodies from earlier infections or vaccination.
Variant surface glycoprotein: The dense surface coat protein expressed by African trypanosomes, which occurs in many antigenically distinct forms. Switching this exposed coat lets the parasite escape antibodies targeting its current form.
Borrelia recurrentis: A bacterium transmitted by body lice that causes louse-borne relapsing fever. Sequential changes in its variable major proteins produce recurring waves of bloodstream infection.

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