immune privileged site 

IMMUNOLOGY

A site in the body in which antigen-T cell interactions facilitate immune tolerance as opposed to immune destruction, which is the norm in non-privileged sites.

Mechanisms of immune privilege

• Physical–paucity or absence of lymphatics

• Low expression of classical MHC class Ia antigens

• High expression of immunoregulatory nonclassical, MHC class Ib antigens

• Overexpression of surface molecules that inhibit complement activation

• Local production of immunosuppressive cytokines–e.g., TGF-β

• Expression of Fas ligand that controls Fas-expressing cell entry

Sites of immune privilege Eye, placenta and fetus, testis; the brain was formerly regarded as a site of immune privilege.  

Reference http://en.wikipedia.org/wiki/Immune_privilege