Immunity in the elderly: the role of the thymus.

Adjustments to lifestyle including social and medical changes have led to human populations having increased longevity in many countries, producing shifts in the population demographics. Approximately half of the increase in the world's population by 2050 may be accounted for by the prolonged survival of those over the age of 60. It is possible to age in relatively good health, but this is rare and for the majority of individuals, growing old is associated with functional impairment, an increased risk of developing a degenerative condition, an increased susceptibility to disease and an increased risk of death. The ageing human population is one of the most urgent challenges facing us today. Changes in the immune system are considered to have a critical role in the decline seen with age, since many infectious diseases may no longer kill an individual, but may contribute to more subtle overall changes. So the impact of infections in older individuals should not be measured only in terms of direct mortality rates, but also by their contribution to the 'indirect' mortality rate and to changes in the quality of life. Taking a pragmatic approach, we need to understand the drivers for immune decline if we are to consider intervening therapeutically in this process. One of the central drivers to this process is age-linked atrophy of the thymus and reversal of this process may have a considerable role in reversing immune decline. Copyright 2009 Elsevier Ltd. All rights reserved.