The corneal homograft reaction, like the rejection of other homotransplanted tissues and organs, is an immune response of the host to the donor cornea. The precise nature of this immune response, however, is not entirely clear.
At the risk of oversimplification, it is useful to divide all immunological reactions into two classes. The first includes all states of hypersensitivity which seem to have their basis in immunologically competent, bloodborne, mononuclear cells. This group of immune phenomena is referred to as delayed hypersensitivity. The second general class of immunological reactions is mediated by circulating humoral antibodies; this group is referred to as immediate hypersensitivity.
We may assume, therefore, that the homograft reaction in the cornea or elsewhere has one of three etiologic mechanisms. First, the antigen of the graft may stimulate the production of immunologically competent lymphoid cells which, in turn, leave their site of origin and enter the grafted foreign
LEIBOWITZ HM, ELLIOTT JH. Antimetabolite Suppression of Corneal Hypersensitivity. Arch Ophthalmol. 1965;73(1):94-111. doi:10.1001/archopht.1965.00970030096022