Donated blood is routinely classified by type as either A, B, AB, or O and as Rh positive or Rh negative. The immune system normally defends the body against potentially harmful substances (antigens). When foreign antigens enter the body, they trigger an immune response in which antibodies are made to help destroy the invaders.
The surface of red blood cells has several proteins that the body can identify as antigens. Two of these antigens are referred to as A and B. Blood is typed based on the antigens present in the blood (A, B or AB). Type O blood has neither the A nor B antigen. Also, each blood type has an antibody against the opposite antigen. So, for example, people with Type B blood have antibodies against the A antigen.
Blood also contains another group of antigens called rhesus factors (Rh factors). People with Rh factors are Rh positive and people without are Rh negative.
In the past, people with Type O/Rh-negative blood were considered universal blood donors — because their red blood cells had neither AB or Rh antigens. This implied that anyone, regardless of blood type, could get Type O/Rh-negative blood without having a transfusion reaction.
But scientists now have a much better understanding of the complex issues related to reactions to incompatible donor blood cells. Even donors with Type O/Rh-negative blood may have other antibodies that cause serious reactions.
Before a blood transfusion, small samples of the donor's blood and the recipient's blood are mixed to check compatibility (crossmatched). Despite these precautions, a transfusion recipient may have antibodies against the donor's red blood cells, white blood cells or platelets. Signs and symptoms of an adverse reaction include:
- Chills
- Fever
- Hives
- Wheezing
- Shock
- Kidney failure
