Very often, when some human organs like heart, eye, liver, kidney fail to function satisfactorily, transplantation is the only remedy to enable the patient to live a normal life.

Then a search begins-to find a suitable donor.

Grafts from just any source-an animal, another primate, or any human beings cannot be made since the grafts would be rejected sooner or later.

Tissue matching, blood group matching (histocompatibility) are essential before undertaking any graft/transplant and even after this the patient has to take immuno-suppresants all his/her life. The body is able to differentiate 'self ' and "nonself' and the cell-mediated immune response is responsible for the graft rejection.

In an identical twin there is no chance of rejection of transplanted organ, so it is advantageous.