For an organ transplant, it is an advantage to have an identical twin. Why?

Explanation

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.