Despite the relatively high success rate and the word 'immunologically privileged site', it has been known that a corneal graft can induce an allograft rejection reaction. This is especially true in the rat where orthotopic penetrating corneal grafts in certain strain combinations are rejected even when transplanted in avascular bed. Reliable microsurgical techniques, together with the availability of inbred or congenic strains and a rapidly developing knowledge of its major histocompatibility complex (MHC) and immune system in general, have made the rat a prime species in which to study the immunological events after corneal grafting. This review describes recent progress in understanding the immunological mechanisms behind corneal graft rejection. The topics discussed include the rat MHC (RT1) antigens and their distribution in the cornea; different responder status in fully allogeneic strain combinations, including the importance of multiple non-MHC antigens; and the role of antigen-presenting cells (APCs) in corneal graft rejection.

SUMMARYThe success rate of corneal transplantation is very similar to that of other organ transplantations because corneal transplants can induce an allograft rejection similar to other organ transplants. Only the centre of the cornea can be considered an immunologically Medawar l -3 reported the histocompatibility relation ships between donor and recipient and the role of specific immunity in heterotopic skin graft rejection by using the anterior chamber of the eye. Medawa r described on the basis of the immunised rabbit model that, 'a skin homograft survives transplantation to the eye of a specifically and strongly immunised rabbit if, and only if, it remained unvascularised. The experiments provide further evidence for the general rule that transplantation immunity is the outcome of a systemic and not a local reaction; and they offer a rational explanation for the well-known clinical success of corneal homografts in human beings. ' However, the clinical demonstrations by Maume nee 4--6 that the corneal graft might be subject to immunological rejection, and that this process might account for a substantial proportion of those cases of late clouding of corneal allografts, prompted numer ous investigators to investigate each of the factors which might account for immunological privilege where it exists and for specific graft rejection when it occurs.Nowadays, recent improvements in microsurgery for orthotopic corneal transplantation in the rat (in

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