Blood type must be determined before blood is transfused into a patient to prevent a transfusion reaction.
When someone has type A blood (with type A antigens), their plasma has type B antibodies and vice versa. Therefore, a reaction occurs when the antigens on the red blood cells of the donor blood react with the antibodies in the recipient's plasma.
For example, if a unit of blood type A (which contains A antigens) is transfused into someone with type B (they have anti-type A antibodies in their blood), a transfusion reaction will occur.
When a transfusion reaction does occur, an antibody attaches to antigens on several red blood cells. This causes the red blood cells to clump together and plug up blood vessels. Then the cells are destroyed by the body (a process called hemolysis), releasing hemoglobin from the red blood cells into the blood. Hemoglobin is then broken down into bilirubin, which can cause jaundice.
If an emergency blood transfusion is needed and the recipient's blood type is not known, anyone can receive type O- blood. Type O- blood (which has no antigen on its surface) will not react with antibodies in the recipient's plasma. Anyone with type O- blood is called a universal donor. Those with type AB blood (which has no antibodies) are called universal recipients, because their plasma will not react with donated blood.