In the corneal grafting, the immunological reaction is often existent and although it is mitigated, it still represents a major cause in the keratoplasty's failure. The immunologic privilege of the cornea it's because of the features of the grafting on one hand and because of the features of the receiving cornea, on the other hand, its essential feature being the absence of blood and lymphatic vessels. It might be difficult to distinguish between an allografting reaction and another cause of failure of the keratoplasty. More clinical elements are still reminiscent for an immunological rejection. The incompatibility HLA is theoretically, the main condition of the allografting reaction. But, for several reasons, the typing of HLA isn't done in the majority of keratoplasties. Some preoperative treatments of the grafting tissues reduce the immunological reaction. In order to prevent the rejection, the corticotherapy is widely used today. The A cyclosporine, a new immunomodulator, seems to be a very interesting progress in the therapeutical means.