Transplant rejection occurs when the body's immune system recognizes a transplanted organ or tissue, such as a kidney or heart, as foreign. This happens because the immune system is designed to protect the body from harmful substances, including bacteria and viruses. When a transplant is performed, the body may attack the new organ, leading to inflammation and damage. There are different types of transplant rejection, including acute and chronic rejection. Acute rejection can happen days to weeks after the transplant, while chronic rejection may develop over months or years. To help prevent rejection, doctors often prescribe immunosuppressive medications that weaken the immune response, allowing the body to accept the new organ.