Activation of microglia: A dangerous interlude in immune function in the brain

Microglial cells are representatives of the immune system in the CNS parenchyma. Their most characteristic property is their ability to modify their behavior in response to diverse signals from other cells in a variety of experimental conditions and human diseases, both acute and chronic. The transformation from a quiescent state into phagocytic brain macrophages is under strict control and accompanied by the production of several secretory products. These include cytokines, excitatory amino acids, and reactive oxygen metabolites by which the activated microglial cells correspond with other cells of the brain and immune system. Thus, they represent an essential host defense and repair system, and may be responsible for tissue destruction and neuronal death, depending on the balance of activating and inhibitory signals.