<p>Unexpected findings have emerged about how and when certain infection-killing white blood cells decide to form memories about their encounters with a pathogen.&nbsp;</p>

<p>These cells can turn themselves into durable memory cells that can survive a long time after an&nbsp;initial infection is cleared. They are&nbsp;prepared to&nbsp;recognize and eliminate future intrusions by the same kind of pathogen.</p>

<p>That is one reason people are resistant to some infectious diseases after exposure to or recovery from the illness. Vaccinations also work this way, by teaching the immune system to spot and attack dangerous viruses, parasites or bacteria.</p>

<p>Researchers have discovered a reversible switch in cytotoxic T lymphocytes, a type of immune white blood cell that directly destroys infected cells. The switch enables cells that have decided to become short-lived effector cells with potent cell-killing ability to reverse course and&nbsp;join the memory cell pool.&nbsp;This flexibility can allow the immune response to be more nimble during the evolving circumstances of an infection.</p>

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