We briefly describe the similarities and differences between a systemic and a local immune reaction and review the evidence that the latter occurs in Alzheimer's disease (AD) brains. The evidence comes mainly from studies on the complement system, microglia, and cytokines, all of which are important actors in the inflammatory process. The evidence is now overwhelming that the complement proteins and many of the mediators of inflammation are produced locally by brain cells. We will mention briefly the many epidemiological studies indicating that the use of anti-inflammatory drugs reduces the incidence and slows the progress of AD. Mention will also be made of some recent work on animal models of possible relevance to AD and inflammation.