Phase variation

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Phase variation is an immune evasion technique employed by various types of bacteria, including Salmonella species. It involves the switching of surface antigens, to evade specific adaptive immune system responses.

Salmonella use this technique to switch between different types of the protein flagellin. As a result, flagella with different structures are assembled. Once an adaptive response has been mounted against one type of flagellin, or if a previous encounter has left the adaptive immune system ready to deal with one type of flagellin, switching types renders previosly high affinity antibodies, TCRs and BCRs ineffective against the flagella.