 A potential problem with embryonic stem cells is that they will normally not be genetically identical to the patient. Embryonic stem cells are typically derived from embryos donated for research after in vitro fertilization (IVF) treatment. Because these stem cells would have a genetic identity different from that of the recipient – the patient – they may, when used in therapy, be rejected by her immune system. Immunorejection can occur when the recipient’s body does not recognize the transplanted cells, tissues or organs as its own and as a defense mechanism attempts to destroy the graft. Another type of immunorejection involves a condition called graft-versus-host disease, in which immune cells contaminating the graft recognize the new host – the patient – as foreign and attack the host’s tissues and organs. Both types of immunorejection can result in loss of the graft or death of the patient. It is one of the most serious problems faced in transplant surgery.