The transplantation of neural tissue into the brain has the potential to repair the effects of trauma and of congenital abnormalities and to replace tissue that has ceased to function as the result of disease. The brain is not an immunologically privileged site in the strict sense; under certain carefully defined circumstances, grafts can last for long periods of time, but many such grafts are rejected naturally or following various types of provocation. Factors important to the survival of neural grafts are the use of embryonic tissue, the immunogenetic match, the level of expression of major histocompatibility complex antigens in brain cells close to the graft, and the presence of neural degeneration near the graft. Much additional basic experimental work must be done, however, before a rational approach to the clinical use of neural grafts can be undertaken.
Gill TJ, Lund RD. Implantation of Tissue Into the BrainAn Immunologic Perspective. JAMA. 1989;261(18):2674–2676. doi:10.1001/jama.1989.03420180098038