he classical view of 'immunosenescence' has envisaged a generalized, inexorable, agerelated, unidirectional decline in immune responses. However, recent studies indicate that nearly every component of the immune system undergoes dramatic age-associated restructuring, leading to changes that include enhanced as well as diminished function. Indeed, the emerging consensus is that immunological ageing is part of a continuum of developmental processes, encompassing complex reorganizational events, compensatory mechanisms and qualitative alterations in function.
Among the most dramatic health outcomes of the ageing immune system are the increased rates of morbidity and mortality that are due to infection. Related effects include diminished protective immunity following prophylactic influenza vaccines, blunted reactivity to diagnostic Tuberculin skin tests and re-emergence of such latent infections as Varicella zoster 1 . Although in vitro models of T-cell ageing provide some insight into the cellular dynamics involved in the immune control over infections and cancer, information on changes at the level of single cells must be merged with data on the interaction of these cells with the in vivo aged environment, taking into account such global ageassociated changes as those that occur in proteasome function, membrane viscosity, lymphocyte homing, altered redox balance and response to physiological stress. The purpose of this article is to provide an overview of the newly emerging paradigm regarding the complexity and multifaceted effects of ageing on the immune system.
Lymphocytes in the peripheral lymphoid tissues Shifts in cell population profiles