BLOOD TYPES & TRANSFUSIONS
Blood is grouped into four types: A, B, AB, and O. Each type is also classified by an RH factor either positive ( + ) or negative ( - ).
When a blood transfusion is necessary, donor and patient blood must be compatible. 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.