This new view of the immunoreactivity of the normal brain is based on three key components. First, there is an active and highly-regulated communication between the brain and the central immune organs. Secondly, the connection from the brain to the draining nodes is much larger than previously appreciated. And third, the blood-brain barrier, by virtue of its selective permeability properties, contributes to the regulation of immunoregulatory cells and molecules in the brain cell microenvironment.

The brain has been characterized immunologically as a site of limited reactivity. This concept, termed immune privilege, was developed from classical studies which showed that allografts usually fare better in the brain than in more conventional sites. Immune rejection of transplants is usually a celbmediated immune reaction. Mechanisms of humoral immunity in the normal central nervous system (CNS) and their relation to the concept of immune privilege were not defined. The biological significance of immune privilege in the brain and the eye, which includes an extracranial projection of the CNS (the retina), seems clear. As expressed by Leslie Brent,'it may be supposed that it is beneficial to the organism not to turn the anterior chamber or the cornea of the eye, or the brain, into an inflammatory battlefield, for the immunological response is sometimes more damaging than the antigen insult that provoked it'.