The immunogenic privilege of corneal allografts.

Corneal grafts enjoy the highest success rate of any form of organ transplantation. The low incidence of graft rejection is particularly impressive considering that human leukocyte antigen matching of donor and recipient is not normally performed. Although corticosteroids are applied topically, systemic immunosuppressive drugs are not routinely implemented for keratoplasty. The most widely accepted explanation to account for corneal allograft success suggests that the avascularity of the graft bed prevents corneal alloantigens from reaching the regional lymphoid tissues and therefore results in an "afferent blockade" of the immune response. However, recent findings suggest that the unique immunologic characteristics of the corneal graft itself may play a crucial role in determining the fate of the transplant. In particular, the presence and distribution of donor-derived Ia+ Langerhans cells can have a profound impact on graft immunogenicity and thus, graft survival even if the graft bed is initially free of lymphatic and blood vascular drainage channels. Thus, the immunologically unique characteristics of the corneal graft conspire with the avascular graft bed to produce an "immunologically privileged" environment that promotes graft survival.