The central nervous system is an immune-privileged site because of its limited powers of regeneration. However, in situations of either systemic or local immune stimulation the CNS can induce inflammatory reactions.
Microglia and perivascular macrophages are the only permanently resident immune cells in the brain of which the microglia constitute the largest population constituting 20% of the non-neuronal population and derive from a distinct population of CD45+ monocytic cells that migrate into the brain during early embryogenesis. With their 3-D structure of their long fine motile processes, they are responsible for immune surveillance. However, they also play an important role in developmental processes such as neuronal differentiation and synaptic pruning. Prolonged pro-inflammatory activation state of microglia is linked to neurodevelopmental disorders and neurodegeneration. Thus, the challenge is to return pro-inflammatory microglia to a resting state without inducing acquired deactivation preventing normal activation.