Transplant rejection is a common complication following organ transplantation, where the recipient's immune system recognizes the transplanted organ as a foreign entity and attacks it. There are two main types of transplant rejection: hyperacute rejection, which occurs immediately after transplantation and is typically due to pre-existing antibodies in the recipient's blood, and acute rejection, which occurs days to months after transplantation and is the result of cellular immune responses. Chronic rejection may also occur over time and can lead to long-term damage of the transplanted organ. Various immunosuppressive medications are used to prevent or treat transplant rejection, but they can have side effects and complications. Research in this area focuses on understanding the immune responses involved in transplant rejection, developing new strategies to prevent rejection, and improving long-term outcomes for transplant recipients.