Elevated levels of inflammation characterize the aging brain.This increase appears to be intrinsic since it is present in the absence of any exogenous provocative stimulus such as infection or trauma.It is likely to represent failure of the immune system to return to low basal levels after the ending of a provoking stimulus.In time, this failure to down-regulate can lead to a gradual accumulation of persistent responses to prior insults, and this can lead to extended immune activation.