Infectious diseases and aging: immunologic perspectives.

Four theories of aging provide a basis for understanding both the aging process and the primary alterations in host defenses that may occur with age. The immunologic theory of aging may be especially important in this regard, since immunologic defects may be responsible for many manifestations of aging, and a decline in immune function may also underlie the increased susceptibility of the aged to infection. However, multiple factors are probably responsible for the aging process and for changes in host defenses. Although several studies attempting to enhance the immune response of elderly persons and perhaps extend lifespan are in progress, no factor at present appears to be as important in this regard as three underutilized immunizations for influenza, pneumococcal infection, and tetanus.