Why do we reject grafts?

One of the puzzles in clinical and experimental organ and tissue transplantation has been why it is that, with the exception of transplantation between genetically identical twin pairs, transplantation results in a rapidly progressing immune response to the point of tissue necrosis in a matter of a few days, even though the recipient has not been immunized to the donor. Indeed, the numbers of T lymphocytes predetermined to react against grafts are relatively abundant compared to those that recognize microbial antigens. Cellular, molecular, and genetic studies are now revealing the nature of T-cell recognition, and why it is that we cannot freely exchange tissue grafts.