Privileged Sites

J Wayne Streilein, The Schepens Eye Research Institute, Department of Ophthalmology, Harvard Medical School, Cambridge, Massachusetts, USA

Copyright © 1998 Elsevier Ltd. All Rights Reserved.

Certain anatomic sites and/or tissues of the body exhibit the property of immune privilege: grafts of histoincompatible tissues placed within these sites are either not rejected at all, or only very slowly. Sites which have been considered to be privileged include brain, eye (anterior chamber, vitreous cavity, cornea, subretinal space), testis and ovary, adrenal cortex, hamster cheek pouch, and the pregnant uterine decidua and the fetal-placental unit. Privilege is usually extended permanently to grafts that are weakly antigenic, i.e. that display the products of only minor histocompatibility loci, whereas privilege may be transient if the immunogenetic disparity between graft and recipient includes major histocompatibility complex (MHC)-encoded antigens. The difference in survival times of MHC and non-MHC disparate grafts in privileged sites reveals that the phenomenon has quantitative features, i.e. privilege is not 'all or none'.

Until recently, immune privilege was not thought to be extended to histoincompatible grafts placed in privileged sites of recipients previously sensitized to antigens expressed on the graft. However, it now appears that there are factors operative within immune privileged sites that can suppress at least certain components of the alloimmune rejection response, even after they have been generated. Thus, immune privilege is a process that, on the one hand, alters the induction of immunity to antigenic materials first encountered via privileged sites, and, on the other hand, inhibits the expression of certain forms of alloimmunity in these same sites.

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