The hypothesis of a blood-brain barrier which prevents substances from entering the brain from the general circulation has been investigated frequently since 1885, when Ehrlich observed that certain aniline dyes injected into the blood stream stained all the tissues of the body except those of the central nervous system. Similarly, the growth of tumor heterografts1 or skin homografts2 has given rise to the concept that a barrier also prevents substances in the brain from entering the general circulation. The brain is, in other words, an "immunologically privileged site."
This immunologic privilege of the brain is based partially on the fact that the brain lacks lymphatic drainage, and, therefore, foreign tissues implanted in the brain are unable to evoke an immune response in their host. The absence of lymphatic drainage would explain the rarity of distant metastases of primary brain malignancies, as well as the response of the reticuloendothelial
IMMUNOLOGICALLY PRIVILEGED?. JAMA. 1964;190(2):150. doi:10.1001/jama.1964.03070150060015