Immunology

Professor Owen made an astute observation: In outbred cattle, dizygotic twins that share the same placenta occur occasionally. Indeed, if one examines peripheral blood samples from such twin pairs, each has blood cells derived from the other, indicating a shared circulatory systems in utero. Surprisingly, even though they are not genetically identical, they accept tissue grafts from one another.

• Inasmuch as these are dizygotic twins and thus are not genetically identical, does their mutual graft acceptance violate the "isografts succeed, allografts fail" rule?
• What does this experiment tell you about the underlying mechanism of that last property of the immune system - non responsiveness to "self?"