Although described for more than 100 years, corneal transplantation has become increasingly common since the 1960s. In 2014, 46,173 corneal transplantations were performed in the United States.  Corneal graft rejection is the most common cause of graft failure in the late postoperative period. 
Examples of corneal graft rejection are shown in the images below.
The term graft rejection refers to the specific immunologic response of the host to the donor corneal tissue. Because it is a specific process, it should be distinguished from other causes of graft failure that are not immune mediated. A corneal graft that has suffered this immunologic response may or may not ultimately fail. Some physicians distinguish between graft reaction, which is reversible with medical therapy, and graft rejection, in which the immunologic end stage has been reached and the process is irreversible. Other physicians simply use graft rejection to refer to this immunologic process at any stage of its development, noting that some cases progress to graft failure because of rejection. This second terminology is used in this article because it is in line with terminology used in other types of organ transplantation.
Furthermore, at the time of presentation, determining with certainty whether an immune process is reversible is impossible.
Ritter et al discuss the need for further study of the genetic modification of corneal grafts prior to surgery to prevent rejection. 
In 2014, 46,173 corneal transplantations were performed in the United States.  The incidence of graft rejection varies widely depending on the study design, type of transplantation, and risk factors for rejection. Reported incidences of penetrating keratoplasty graft rejection range from 2.3% to 68%.  At 5 years’ follow-up in the Cornea Donor Study, 23% of subjects had at least one rejection event, and 37% of the eyes with a rejection event had graft failure. 
Corneal graft rejection is the most common cause of graft failure in the late postoperative period.  The reported incidence of graft rejection is lower in endothelial keratoplasty. Descemet membrane endothelial keratoplasty (DMEK) has been reported to have a rejection rate as low as 0.7% at one year in one series, but other studies have reported higher rates. [6, 7]
Corneal graft rejection is the most common cause of graft failure in the late postoperative period.
No difference in corneal graft rejection between different races is known.
No sex predilection for corneal graft rejection is known.
Host age may influence the risk of corneal graft rejection. Some investigators have concluded that hosts older than 60 years have a lower risk of corneal graft rejection, although this has not been confirmed. The effect of donor age on corneal graft survival has been debated. The Cornea Donor Study did not find an association between donor age and corneal graft survival among corneal transplants at moderate failure risk.  Infants have higher rates of graft rejection than adults.
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