The T cells which mediate immunological memory remain elusive. Identification of such cells would open the door to increasingly specific immunotherapy in areas such as transplantation and autoimmunity. 2. Over the last few years attempts have been made to identify phenotypic markers which can distinguish naive or virgin T cells from primed or memory ones. In humans, great hopes were raised when it was shown that the level of expression of the higher-molecular-mass isoforms (CD45RA) of the tyrosine phosphatase, CD45, correlated with previous exposure to antigen. 4. However, our studies in the mouse and more recent studies in rat and human suggest that expression of CD45RA more closely correlates with the state of responsiveness of the T cell. 5. Thus, with time, activated/memory T cells return to a state of quiescence or hypo-responsiveness and express high levels of CD45RA. Hence, not all CD45RA+ T cells are virgins.
