Immune ageing and Alzheimer's disease.

The implications of immune ageing are considered with respect to Alzheimer's disease. In the course of immune ageing a population of activated autoreactive T cells appears which is proposed to cross the undamaged blood-brain barrier and to initiate overactivation of brain microglia. The development of highly activated microglia appears to be sufficient to induce the inflammatory neurotoxic process characteristic of the brain in Alzheimer's disease. The model thus emerging for sporadic Alzheimer's disease is that of a T cell-mediated chronic autoimmune syndrome following the age-dependent loss of thymic function. This view is consistent with available data and provides testable predictions.