Answer:
Major histocompatibility complex (MHC) antigens
Explanation:
The process in which recipient body rejects the transplanted organ is called a allograft rejection or transplantation rejection.
A transplanted organ contains Major histocompatibility complex (MHC) antigens which are required to be accepted by the recipient's body to function properly but T cells in recipient's body become alloreactive and recognize MHC antigens (donor-derived antigens) as foreign and starts making antibodies against it. Which means the recipient's body is rejecting the  transplanted organ.
Hence, the correct answer is "Major histocompatibility complex (MHC) antigens".
