Decline of the Aging Immune System
An interesting review article can be found at Immunity and Aging: "Longitudinal studies are defining progressive alterations to the immune system associated with increased mortality in the very elderly. Many of these changes are exacerbated by or even caused by chronic T cell stimulation by persistent antigen ... Lifelong exposure to chronic antigenic load is the major driving force
of immunosenescence, impacting on human lifespan by reducing the number of naive antigen-non-experienced T cells, and, simultaneously, filling the immunological space [with] antigen-experienced T cells. Gradually, the T-cell population shifts to a lower ratio of [non-experienced] cells ... the repertoire of cells available to respond to antigenic challenge from previously unencountered pathogens is shrinking. ... cells from old individuals might recognize a limited set of antigens despite being plentiful in number."
Link: http://www.immunityageing.com/content/pdf/1742-4933-3-2.pdf