The answer to this question would be: antibody
There are 4 types of hypersensitivity. In type II hypersensitivity, the reaction depends on the antibody. Some of the body cells surfaces were similar to the pathogens that the white blood cells trying to fight. This causing the body also attack the body cells too. It is also called cytotoxic hypersensitivity.
In mismatch transfusion, the receiver recognizes transfused blood cell as foreign material so it was making antibody to fight it.