T-Cell Cycle and Immunosenescence: Role of Aging in the T-Cell Proliferative Behavior and Status Quo Maintenance Jacek M. Witkowski and Tamas Fulop Contents Introduction ....................................................................................... 2 Basic Features of the T-Lymphocyte Population ................................................ 2 Factors Determining Adequate Strength of the Adaptive Immune Response ................... 3 The Effects and Mechanisms of Aging-Associated Changes .................................... 7 Conclusions and Future Directions ............................................................... 21 References ........................................................................................ 21 Abstract Adequate immunological surveillance and protection by the adaptive (i.e., T- and B-lymphocyte-dependent) immunity depends on the ability of the involved cells to rapidly built sufficient numbers of active effector cells, followed by sufficient numbers of memory cells and inhibitory “regulatory” lymphocytes. This huge proliferative activity assures first that the invading pathogens will be effectively neutralized by the lymphocytes in the shortest possible time. Next, it leaves a permanent trace of each encounter with pathogen in a form of memory cells which would ascertain even quicker and more effective response (and pathogen neutralization) at consecutive encounters. Finally, the proliferative activity and differentiation of specific T-cell subpopulation make sure that the adaptive immune reaction will be terminated when the abovementioned goals are met, in order to avoid excessive consumption of energy and resources and improper activation of the lymphocytes, possibly leading to their impaired functions and J. M. Witkowski (*) Department of Pathophysiology, Medical University of Gdańsk, Gdańsk, Poland e-mail: jawit@gumed.edu.pl T. Fulop Research Center on Aging, Department of Medicine, Immunology Graduate Programme, Faculty of Medicine and Health Sciences, University of Sherbrooke, Sherbrooke, QC, Canada e-mail: tamas.fulop@usherbrooke.ca # Springer Nature Switzerland AG 2019 T. Fulop et al. (eds.), Handbook of Immunosenescence, https://doi.org/10.1007/978-3-319-64597-1_13-1 1