Your immune system protects your body from invaders, like bacteria or viruses. Your immune system can tell the difference between cells that belong to you and cells that don’t belong to you, such as the cells in a donor’s kidney.
Before you get a kidney transplant, doctors do tests to find out if your immune system will react against potential kidney donor cells. If those tests show that you are ‘highly sensitized,’ meaning your immune system will react against the trasplanted organs from 9 out of 10 people who might donate a kidney for transplantation, then it might take a very long time to find a donor whose organ your immune system will accept.
Having a more reactive immune system makes you more likely to experience rejection after the transplant. Rejection is a process in which your immune system recognizes the transplanted organ as foreign and attacks your new kidney.