Classically the brain has been regarded as an ‘immunologically privileged’ site, because alien tissue grafts transplanted there survive longer than similar grafts in other sites (Barker & Billingham, 1977). The relative hospitality of the brain to foreign tissue has been attributed to a lack of lymphatic drainage, the presence of the blood–brain barrier, the lack of constitutive expression of major histocompatibility complex (MHC) molecules, and the possible presence of chemical substances that might inhibit lymphocyte traffic. However, recent studies indicate that, in general, immune responses proceed in the nervous system in a similar manner to that in other organs. Yet the nervous system still has a number of attributes that influence local immune responses and that may be relevant to the pathogenesis of autoimmune neurological disease.

Specialization of structure and function in the nervous system

Central and peripheral nervous system

The nervous system is subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises the cerebral hemispheres, the cerebellum, the brainstem, the spinal cord, and the olfactory and optic nerves. The PNS comprises the cranial nerve roots and cranial nerves, the spinal nerve roots (dorsal and ventral), the dorsal root ganglia, the spinal nerves and the peripheral nerves.