Neural grafts from day 17–19 fetal rats or mice survived well when transplanted into syngeneic, or immunodeficient hosts, thus demonstrating that there are no non-immunological barriers to crossspecies transplantation of neuronal tissue in rats and mice. However, intraventricular grafts from rat to mouse, or vice versa, in immunocompetent animals were rejected in less than 30 days. By this time all graft tissue had been destroyed and scavenged, presumably by the macrophages seen infiltrating the grafts within 10 days of grafting. Rat allografts from major histocompatability complex disparate donors survived well as did grafts between rats differing only at minor histocompatability loci. However, allografts from donors that differed from recipients at both major and minor histocompatability complex loci had a variable survival time. When neural tissue was grafted into immunologically primed recipients, it was rejected as was similar tissue grafted beneath the kidney capsule of an allogeneic host. Concomitant grafting of allogeneic tissue under the kidney capsule and into the third ventricle was followed by rejection in both sites. A striking observation in these studies was the induction of Class I major histocompatability complex antigens on grafted neuronal tissue. High levels of antigen expression were correlated with a vigorous host response and poor graft survival but lower levels were not indicative of impending graft destruction.

Whilst the brain can be regarded as an immunologically privileged site, the privilege is not absolute and caution needs to be exercised in the interpretation of results from allogeneic or xenogeneic grafts.