The Blood–Brain Barriers

The blood–brain barriers (BBBs) are mainly located at the levels of the vasculature, choroid plexus, circumventricular organs, and leptomeninges and play multiple roles in neuroimmunology. The ability of the BBBs to separate the blood and its contents from the central nervous system (CNS) is largely responsible for the CNS being an immune-privileged region. However, the BBB then revises this separation in a regulated way by a variety of mechanisms, including the ability to transport cytokines, control the trafficking of immune cells into the brain, and to itself secrete into the blood and the CNS immunoactive substances. The BBB thereby participates in a number of neuroimmune axes that allow communication between the CNS and the peripheral immune cells. Failure of the highly regulated activities of the BBBs can be both a cause and consequence of immune diseases.