Immune privilege and angiogenic privilege of the cornea.

The cornea is the transparent window of the eye and corneal transparency is essential for good vision. Inflammatory reactions within the cornea cannot only cause tissue destruction and scar formation, but are also associated with angiogenesis and lymphangiogenesis in the cornea. Both inflammation-associated processes interfere with corneal transparency and cause corneal blindness. During evolution the cornea has developed mechanisms for preventing and modulating inflammatory and angiogenic reactions. The fact that the cornea is normally devoid of both blood and lymphatic vessels and actively maintains this avascularity has been termed 'corneal angiogenic privilege'. Corneal 'immune privilege', on the other hand, indicates that the cornea is an immune-privileged site and tissue, enabling the extraordinary success of histologically incompatible corneal transplantation. Recent evidence indicates that there is considerable overlap in the molecular mechanisms maintaining corneal 'angiogenic' and 'immune privilege'.