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 include: 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 include: Immune privilege is also believed to occur to some extent or able to be induced in articular cartilage. 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 CNS contribute to the maintenance of neurogenesis and spatial learning abilities in adulthood. 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 of normally rejected stimuli. 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). Sympathetic ophthalmia is a rare disease which results from the isolation of the eye from the systemic immune system. Usually, trauma to one eye induces the release of eye antigens which are recognized and picked up by local antigen presenting cells (APC) such as macrophages and dendritic cells. These APC carry the antigen to local lymph nodes to be sampled by T cells and B cells. Entering the systemic immune system, these antigens are recognized as foreign and an immune response is mounted against them. The result is the sensitization of immune cells against a self-protein, causing an autoimmune attack on both the damaged eye and the non-damaged eye.