Tissue grafts are normally recognised as foreign antigens by the body and attacked by the immune system.
However, in immune privileged sites, tissue grafts can survive for extended periods of time without rejection occurring.
[2][3][4] This was once thought to also include the brain, but this is now known to be incorrect, as it has been shown that immune cells of the central nervous system contribute to the maintenance of neurogenesis and spatial learning abilities in adulthood.
[8] Antigens from immune privileged regions have been found to interact with T cells in an unusual way: inducing tolerance of normally rejected stimuli.
Other factors that contribute to the maintenance of immune privilege include: The nature of isolation of immunologically privileged sites from the rest of the body's immune system can cause them to become targets of autoimmune diseases or conditions, including sympathetic ophthalmia in the eye.
In both mouse models and humans diminished numbers of Tregs were associated with immunological rejection of the fetus and miscarriage.
[citation needed] Sperm are immunogenic – that is they will cause an autoimmune reaction if transplanted from the testis into a different part of the body.
[20] The likely reason for their immunogenicity or rather antigenicity is that sperm first mature at puberty, after central tolerance has been established, therefore the body recognizes them as foreign and mounts an immune reaction against them.
[25] It is thought that immune privilege within the CNS varies throughout the different compartments of the system, being most pronounced in the parenchyma tissue or "white matter".
[25] Thus, the CNS is thought to be limited in its capacity to deliver antigens to local lymph nodes and cause T-cell activation.
Dendritic cells from cerebrospinal fluid have been found to migrate to B-cell follicles of cervical lymph nodes.
[29] This was seen in the absence of the T-cell mediated inflammatory "delayed type hypersensitivity reaction" (DTH) when the antigen was reintroduced in another part of the body.
[citation needed] There is great potential for use of molecular mechanisms present in immune privileged sites in transplantations, especially allotransplantations.
[30] Research suggests that the exploitation of anterior chamber-associated immune deviation (ACAID), aqueous humor and its anti-inflammatory properties and the induction of regulatory T cells (Treg) may lead to increased survival of allotransplants.
[33] In another study on type II diabetic and obese mice, the transplantation of microencapsulated Sertoli cells in the subcutaneous abdominal fat depot lead to the return of normal glucose levels in 60% of the animals.
[34] The existence of immune privileged regions of the eye was recognized as early as the late 19th century and investigated by Peter Medawar.