Page 1127 - Veterinary Immunology, 10th Edition

VetBooks.ir Corneal Allografts

Certain areas of the body, such as the anterior chamber of the eye,
the cornea, the thymus, the testes, and the brain, are immune-

privileged sites. As a result, grafts made into these sites may not be
rejected. In humans, for example, 90% of first-time corneal
allografts survive without tissue typing or immunosuppressive
drugs. These sites are privileged because the body rigorously
controls inflammation in these critical tissues. Several mechanisms

are involved in this. They have an impermeable blood-tissue
barrier, lack dendritic cells, contain suppressor cells, express low
levels of MHC class I and II molecules, and may contain high levels

of immunosuppressive molecules such as IDO, transforming
growth factor-β (eyes and testes), neuropeptides (eyes),
complement inhibitors (eyes), and corticosteroids (testes).
Molecules found in normal aqueous humor also interfere with
innate immune mechanisms. They block NK cell lysis, inhibit

neutrophil activation by CD95L, suppress nitric oxide production
by activated macrophages, and interfere with alternative
complement activation. The eye and testes are also unique in that
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they express very high levels of CD95L. As a result, any CD95 T
cells that enter these organs will bind to CD95L and be killed by
apoptosis.

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