[The brain and the immune system].

The brain is extremely vulnerable to changes in the local milieu. An immune reaction in the brain can be fatal to the host even if it effectively inhibits an infectious process. However, during development, the brain has become isolated both from invading microorganisms and from the immune system. This isolation is achieved by three important elements: the blood-brain-barrier, the lack of lymphatic drainage from the brain and the absence of MHC molecules on the brain cells. The blood-brain-barrier blocks immune cells and circulating molecules from the immune system from entering the brain. The lack of lymphatics prevents immunogenic material from reaching regional lymph nodes and the lack of MHC molecules makes brain an immunologically inactive milieu. The brain thus lacks some elements of the immune system and does not participate in immune reactions. One consequence of this isolation is that it makes brain vulnerable to infections. The defence systems are so weak that microorganisms that are not regarded as dangerous in themselves may lead to serious consequences if they manage to penetrate the blood-brain-barrier. There are also similarities between the brain and the immune system. Both are composed of cells that interact with each other by means of chemical signals. The two systems apparently also "talk" to each other; the transmitters of the brain may influence the function of the lymphocytes and some lymphokines that are produced during an immune response may also function as signal substances in the brain.(ABSTRACT TRUNCATED AT 250 WORDS)