Whereas skin and many other tissues are destroyed soon after their transplantation from one individual to another, clinical results have suggested that corneal homografts survive their transplantation for long periods, and perhaps indefinitely. The chief among several possible explanations of this apparent anomaly are (
a
) that corneal tissue as such is for some reason unable to provoke transplantation immunity; (
b
) that grafts transplanted to the cornea are either incapable of provoking immunity in that position, or (
c
) that they do so, but are protected from its consequences. These possibilities have been investigated by experiments on rabbits. It is shown that corneal homografts transplanted to richly vascular beds prepared in the skin of the chest, though they heal in, become vascularized and proliferate, are not long tolerated by their hosts. They break down just as skin homografts do when so transplanted. Furthermore, once a host has reacted against corneal homografts transplanted to its chest it becomes so affected as to accelerate the destruction of farther corneal homografts later transplanted from the same donor. It is inferred that when corneal homografts are trans planted in such a manner that they become vascularized, they immunize their hosts just as skin homografts do ; corneal tissue as such is therefore antigenically effective. The possibility that the intact cornea offers an immunologically privileged site for grafts of homologous tissue was investigated by carrying out the converse operation. Minute skin homografts transplanted to small pockets cut in the corneas of animals which had previously been immunized against their donor’s skin long outlived skin homografts transplanted on the same occasion to the recipients’ chests, provided that the grafts in the corneas remained un-vascularized. If, however, vessels from the limbus grew out and penetrated the grafts, they promptly broke down. It is inferred that tissue transplantation immunity is ineffective within the substance of the cornea so long as it remains avascular. The question of whether homografts transplanted to the cornea can themselves provoke immunity is thus of no practical importance, since any such immunity would be quite ineffective.
Biologia futura: embryo–maternal communication via progesterone-induced blocking factor (PIBF) positive embryo-derived extracellular vesicles. Their role in maternal immunomodulation
