Protective Immunitv: It 1s well estahlished that T cell memory, as assessed by accelerated recall responses in vivo, is long-lived (5). However, the nature U nd e rsta nd i n g The i r Reia ti on of T cell memory has remained controversial, with dehate centered around two o~? - Rafi Ahmed* and David Gray The immune system can remember, sometimes for a lifetime, the identity of a pathogen. Understanding how this is accomplished has fascinated immunologists and microbiologists for many years, but

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... there is still considerable debate regarding the mechanisms by which long-term immunity is maintained. Some of the controversy stems from afailure to distinguish between effector and memory cells and to define their roles in conferring protection against disease. Here the current understanding of the cellular basis of immune memory is reviewed and the relative contributions made to protective immunity by memory and effector T and B cells are examined.