Changes in T Cell Homeostasis and Vaccine Responses in Old Age.

T lymphocytes follow three main stages of differentiation during their lifetime history - memory generation, memory homeostasis, and immunosenescence. Current definitions of T cell immunosenescence now include distinct aspects of both T cell senescence and exhaustion. Multiple studies have indicated a loss of vaccine efficacy in old age, and because this period coincides with the onset of T cell immunosenescence, the latter has often been implicated in the loss of vaccine responsiveness. This chapter examines changes in T cell homeostasis with age, and proposes mechanisms of how these changes, together with senescence and exhaustion, could affect the T cell contribution to the vaccine response.