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All organs have a specific antigen kind of like a uniqye brand on each organ. This also differes from person to person. Therefore, when transplanting an organ into a different body, the reciever's white blood cells do not recognise the unfamiliar antigen. This leads it to believe that it is a foreign object trying to attack the body and therefore the white blood cells attack the transplanted organ. This is when the body rejects the organ.

There are some ways of reducing the risk of rejection such as getting an organ that is as close to the reciever's antigens as possible as well as providing drugs called immunosuppressants which are used to stop the white blood cells from attacking the organ.