Answer:
The immune System
The immune system never rests—its cells constantly patrol the circulation. Without the immune system, the body would be overwhelmed with infections. With it, blood transfusions must be performed with great care.
If incompatible blood is given in a transfusion, the donor cells are treated as if they were foreign invaders, and the patient's immune system attacks them accordingly. Not only is the blood transfusion rendered useless, but a potentially massive activation of the immune system and clotting system can cause shock, kidney failure, circulatory collapse, and death.
