Cornea transplant in humans is almost never rejected.This is because
1) it's composed of enucleated cells
2) it's a non-living layer
3) it's cells are least penetrable by bacteria
4). no blood supply
answer is no blood supply. Because of the avascular nature of cornea, when we transplant it is not involved in any interaction . no lymphocytes gather over there and no phagocytosis.