Critical period is the first 2-4 weeks. Pancreatic islets are rejected even with strong immunosuppressive treatment. In rat, injection of pancreatic cells directly into the thymus of a diabetic recipient, allowed the long term survival of the grafted cells. It is possible that the developing thymocytes became tolerant to the alloAgs on the islet cells. They are privileged due to the absence of lymph vessels and sometimes even blood vessels.
Cornea transplants are very successful.
Transplantation of fetal brain-stem neurons into patients with Parkinson is under trial. Sequestration of Ags might be obtained by encapsulating the cells. Pancreatic islet cells were encapsulated into semipermeable membranes and transplanted into diabetic mice. They survived and produced insulin. Will aleviate the problem of lack of human transplantable organs.