There are several different types of transplantation. An autograft is a graft from one part of the body to another site on the same individual. An isograft is one between individuals that are genetically alike, as in identical twins. An allograft is a graft between members of the same species but who are not genetically alike. A xenograft is one between members of different species. The allograft we are most familiar with is that of a blood transfusion. Nonetheless, the replacement of diseased organs by transplantation of healthy tissues has frustrated medical science because the immune system of the recipient recognizes that the donor organ is not "self" and rejects the new organ.

The ability to discriminate between self and nonself is vital to the functioning of the immune system so it can protect the body from disease and invading microorganisms. However...