ABSTRACT
Throughout the body, but mainly in the periphery, adaptive and innate immune systems operate to help protect against a variety of physical and biological insults. The brain, however, is in a slightly different situation because it is encased by the skull. For many years it was thought that the brain was an organ privileged against both adaptive and immune reactions, so that adaptive immune system mediated inflammatory swelling occurs rarely, especially when and where there is an intact blood-brain barrier. The concept that the brain is immunologically privileged does not hold, however, for the innate immune system, which has been implicated increasingly in a variety of acute and chronic neurodegenerative disorders. The cells that mainly constitute the innate immune system in brain are phagocytic microglia, and in addition, astrocytes, endothelial cells, and neurons; all contribute to neuroinflammatory reactions.
