Understanding why we age and how that process can be delayed or reversed are topics that have engaged both researchers and the general public [1]. From a scientific perspective, aging is a fascinating biologic question, and knowledge of this process will undoubtedly provide insights into fundamental cellular mechanisms. The subject also intrigues us all because aging affects everyone. If we can find strategies to improve the health of the aged, it would have enormous societal implications, since meeting the medical needs of the growing population of elderly is a huge challenge.

Aging affects multiple organs, but its effects on the immune system are among the best studied. One reason for this is that developing and mature lymphoid cells are easily isolated and manipulated in developmental and functional assays in comparison to cells from other tissues. Furthermore, as pointed out by Liu and Sharpless, lymphoid cells persist for extended periods and thus are particularly susceptible to environmental insults that may lead to age-associated defects. These are being increasingly defined. For example, there is a general consensus that declines in immune function with age make the elderly more susceptible to infectious agents, such as influenza, with increasing morbidity and mortality. Furthermore, as reviewed by McElhaney and Effros, vaccination is less efficient in aged individuals. Thus, a better understanding of age-induced immune senescence may lead to the development of strategies to circumvent impaired immunity and other limitations of aging and to improve human healthspan.