Peripheral CD4÷ lymphocytes can be subdivided into naive, effector and memory subsets (T,, ble I). Naive T cells are resting pr,: cursors which have not encountered antigen (Ag) since their exit from the thymus. Both effector and memory cells are Ag-experienced. It is useful to think of effectors as short-lived, highly activated cells that are specialized to carry out particular functions with great efficiency. CD4÷ effectors include helpers with a high capacity for secreting cytokines when stimulatod by Ag-bearing B cells. Memory cells can be defined as res': ng cells which mediate Agrecall responses. Here we will focus on the features of memory cells that can make this possible.

Frequency and longevity of memory T cells after priming The frequency of T cells specific for previously encountered Ag is substantially increased by exposure. We find a 20-50-fold increase in Ag-specific CD4* T cells after immunization with a soluble protein, I~ LH t. Thus, naice T cells proliferate extensively during their encounter with Ag, and an expanded pool of memory T cells develops and persists when the transient effectors have come and gone. Although specific immunity following an infection can last for years or even a lifetime, some pathogens/vaccines induce only short-term protection. The physical properties of Ag, in vivo persistence, route of exposure, dose and frequency of reexposure can influence the length of immunity 2. In mice, memory T-cell function is found in a longlived population of cells that once generated, is unaffected by adult thymectomy which depletes c, aive cells-3. However, recent studies suggest that individual memory T