Vaccination is today considered as the most effective method of preventing infectious diseases. A vaccine elicits the generation of memory lymphocytes reacting against a specific pathogen. Two subsets of memory T lymphocytes have been identified in humans. These subsets display high functional heterogeneity and assert distinct critical roles in protective immunity. However, the biochemical mechanisms controlling their functional properties remain poorly investigated, mainly due to the low frequency of memory cells and the limited amount of human specimen available. In this book, proteomic and cytosolic transduction approaches have been successfully combined to unravel the biochemical complexity of the signalling pathways of these subsets. Crucial differences in the composition of their signalling machinery were identified, and the different expression of a key regulatory protein found responsible for their functional heterogeneity. This book should be especially useful to professionals in immunology and vaccine fields, or anyone else who may be considering utilizing an array strategy for protein profiling.