The immune status of the central nervous system

The central nervous system (CNS) has no true lymphatic outflow and is protected by the blood-brain barrier. It shares special relationships with the immune system. Though heterologous grafts can sometimes survive inside the brain, explaining that it has been considered as an immunologically privileged site, the CNS is able to generate immunological immune reactions and can exert a regulatory role on the extracerebral ones. Inside the brain, the immunological reactions are probably due to populations of immunocompetent cells and potentially macrophagic cells which are there permanently but in an inactivated state. Nevertheless the first event triggering these reactions and responsible for the activation of immunocompetent cells remains highly hypothetical. The astrocyte-endothelial cells complex which is the anatomical support of the blood-brain barrier probably plays a vital role in this process. Permanent exchanges between the CNS and the immune system are assumed by soluble mediators: lymphokines, neurotransmitters and hormones. These allow the CNS to control to some extent the extracerebral immunological reactions by feed-back regulation mechanisms.