Our immune system recognizes the body's own cells and does not react to them. However, when cells from another person (blood, organs, etc.) enter the body, the system immediately identifies them as foreign and initiates a defensive response to expel them. Similar to how the body reacts to microbes, the same defense mechanism is triggered against a transplanted organ. This reaction by the body, which damages the transplanted organ, is called "tissue rejection" or "rejection."
Tissue rejection is a serious condition that can occur in organ and tissue transplants. If not diagnosed and treated early, it can lead to the transplanted organ or tissue losing its function (death). To prevent tissue rejection, organ transplant recipients use immunosuppressive drugs lifelong to suppress this immune system response. This process is called "immunosuppression."
Despite these medications, tissue rejection can sometimes develop. This condition is recognized by the impairment of the transplanted organ's (e.g., liver's) functions and the detection of findings related to tissue rejection in biopsies. Such an acute rejection episode is usually treated successfully, and the patient's condition returns to normal.