Innate Immunity in the CNS: Redefining the Relationship between the CNS and Its Environment

The concept of the CNS as an immune-privileged organ has led to a common misunderstanding that it is not an active immunological organ, guarded from its surroundings by the blood-brain barrier (BBB). Recent advances in this field clearly demonstrate that the CNS is a highly immunologically active organ, with complex immune responses mostly based on innate immune processes. Such responses implicate a continuum of heterogeneous cell types both inside the CNS, in the periphery, and at their interface, the BBB. This Review aims to discuss the importance of the BBB as the first line of defense against brain infections and injuries of the CNS and the main molecular mechanisms involved in the control of the innate immune system of the CNS. We also review the central role of the neurovascular unit in diseases of the CNS and how it can be targeted for novel therapeutic strategies.