Organ transplant rejection occurs when the recipient's immune system identifies the transplanted organ as foreign and attacks it. This response can happen because the body recognizes the new organ, such as a kidney or heart, as different from its own tissues. Rejection can be acute, occurring soon after the transplant, or chronic, developing over time. To prevent rejection, doctors often prescribe immunosuppressive medications that help the body accept the new organ. These medications lower the immune response, reducing the risk of rejection but also making the patient more susceptible to infections. Regular monitoring is essential to ensure the transplanted organ remains healthy.