My daughter has started to reject her kidney after 12 years of transplant why is this?

Kidney rejection. Your question is best put to the transplant nephrologist handling her case. Due to her having her transplanted kidney for 12 years, the most likely cause is chronic (c) rejection that takes place over many years. In c the body's constant immune response against the new organ slowly damages the transplanted tissues or organ. Speak to her doctor for more information. .