T-cell immunity in the aging human

Immunosenescence is an ill-defined term commonly used to describe the differences observed between younger and older individuals in a broad range of immune parameters. Humans and mice are the best-studied, but indications of immunose-nescence have also been reported in primates, dogs, cats and horses.1 Although often referred to as age-associated changes to immunity, and almost always assumed to be detrimental to immune responses, in most cases, the differences between old and young are neither known to be detrimental nor demon-strated to have changed with age in that individual. This would require longitudinal studies of the same individuals over time, which are clearly challenging in long-lived species like humans, but are also quite rare even in mice, mostly due to cost issues. Although there are, in fact, numerous longitudi-nal studies being conducted in different human populations, in