An immunologically privileged site is any of those locations in the body--the brain, anterior chamber of the eye, testis, renal tubule, uterus, and possibly joints and adrenal glands--where immune response to antigens are not destructive to tissue or is suppressed. It is necessary due to unique self-proteins that reside only in particular tissues and do not travel through the lymphatic system. Examples of tissue-specific proteins include myelin of the central nervous system. Such proteins are sequestered in organs and extracellular fluid does not travel the lymphatic system, meaning they do not come into contact with T cells during negative selection in the thymus. Allografts to these sites are not rejected.

Immune response is modified by the privileged sites through anti-inflammatory cytokine release of TGF-beta, which suppresses immune responses likely to cause tissue damage (favors B-cell antibody release vs. macrophage activation). Cells express FasL on their surface, which binds to cytotoxic T cells and causes the cytotoxic T cells to apoptose rather than destroy the tissue of privileged sites.

Medically, a cornea transplant is currently the only procedure that takes advantage of this.

References

• http://microvet.arizona.edu/Courses/MIC419/Tutorials/autoimmunity.html
• http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=33671