<qa>  What is the main factor reducing graft survival?Immune rejection episodes after penetrating keratoplasty occur in 30 of patients and constitute one of the main factors reducing graft survival. They mainly occur during the first 18 months. Prevention usually relies on a topical treatment with dexamethasone or prednisolone for standard risk patients. Eye drops are instilled three times a day during at least 3 months then tapered. 