If you take an organ from one person and put it in another, if their MHC molecules don't match then the immune system recognizes the immune system of the host recognizes 'this is not the right MHC for me' and the immune tries to destroy those cells.

如果将器官从一个人移植到另一个人,免疫系统识别出MHC分子并不匹配,宿主的免疫系统就会说,这不是我的MHC,免疫系统就会试着摧毁移植器官的细胞

耶鲁公开课 - 生物医学工程探索课程节选