Immune privilege

Certain sites of the human body have immune privilege, meaning they are able to tolerate the introduction of antigens without eliciting an inflammatory immune response. Tissue grafts are normally recognised as foreign antigen 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. Immunologically privileged sites were thought to include:

Immune privilege is also believed to occur to some extent or able to be induced in articular cartilage.

Immune privilege is thought to be an evolutionary adaptation to protect vital structures from the potentially damaging effects of an inflammatory immune response. Inflammation in the brain or eye can lead to loss of organ function, while immune responses directed against a fetus can lead to miscarriage.

Medically, a cornea transplant takes advantage of this, as does knee meniscal transplantation.

Antigens from immune privileged regions have been found to interact with T cells in an unusual way inducing tolerance as previously opposed to a destructive response. Immune privilege has emerged as an active rather than a passive process.

Physical structures surrounding privileged sites cause a lack of lymphatic drainage, limiting the immune system's ability to enter the site. 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.

As well as the mechanisms that limit immune cell entry and induce immune suppression, the eye also contains active immune cells that act upon the detection of foreign antigens. These cells interact with the immune system to induce unusual suppression of the systemic immune system response to an antigen introduced into the eye. This is known as Anterior Chamber Associated Immune Deviation (ACAID).

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