Elena is undergoing treatment for end-stage renal failure and requires a kidney transplant. Her fraternal (non-identical) twin sister, Chloe, is a potential donor. Although Chloe is a closer genetic match than an unrelated donor, Elena's medical team notes that she will still require lifelong immunosuppressant therapy post-surgery.
Explain why a kidney transplant from a fraternal twin still carries a risk of organ rejection, whereas a transplant from an identical twin would not.