Rejecting corneal graft. Close-up of a man's eye showing tissue rejection of a corneal graft. The grafted tissue appears cloudy. Note the zigzag of stitches holding it in place. The cornea is a thin transparent membrane over the front of the eye. It acts like a lens, focusing light, and protects the structures below. Corneal grafts are carried out when the cornea is damaged or diseased but the rest of the eye is healthy. The donor tissue most often comes from another person, but the rejection rate is low because the cornea lacks blood vessels (and hence lacks white blood cells). This graft is eight months old and has begun to reject despite the use of oral and local steroid drugs.