In this study, the investigators will evaluate whether CD4+ TCM producing effector cytokines can be distinguished on the basis of their expression of the IL-7 receptor alpha-chain (CD127). Using CD154 production as a marker of Ag-specific CD4+ T cells, the investigators will also test the hypothesis that the phenotype and function of TCM are influenced by the type of Ag they recognize. TCM specific for two cleared protein Ag, tetanus toxoïd (TT) and hepatitis B surface (HBs), inducing an early stage of CD4+ T cell differentiation will be compared to TCM specific for cytomegalovirus (CMV), a persistent virus inducing an advanced stage of CD4+ T cell differentiation.