Congenitally athymic (nude) mice accepted for their lifetime intact skin grafts from distantly related mammals (cat, human) and birds (chicken). They also failed to immunologically reject skin grafts from reptiles (lizards) and amphibians (tree frog), although the skin in these grafts underwent varying degrees of disorganization. A definitive role for the thymic defect in this failure to reject xenografts was established by showing that thymus implantation into nude mice enabled them to reject such foreign skin.

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