Scientist Karl Landsteiner reported in 1901 that blood could be classified into "types." By matching these blood types, a successful blood transfusion could be made between a healthy donor and a patient in need of blood replacement due to an injury, disease or surgery.  A blood transfusion can either be whole blood or a blood component (red cells, platelets or plasma).

Blood types are based on specific proteins called antigens that are found on the surface of red blood cells, and antibodies found in plasma.  Antibodies can recognize markers on foreign cells (those that are not the body's own cells). When the blood of two people mixes during a transfusion, the antibodies will act against any cells bearing the wrong marker.

The antibody reaction that occurs when two different blood types are mixed causes the foreign red cells to be destroyed. This is called hemolysis, and can lead to kidney damage and death. This is why it is so important to match blood types between a donor and patient before a transfusion is given.

There are four basic blood types:

-Type A with A antigen on the red cells and anti-B antibodies in the plasma.

-Type B with B antigen on the red cells and anti-A antibodies in the plasma.

-Type AB with both A and B antigens on the red cells and neither anti-A nor anti-B in the plasma.

-Type O with no A or B antigens on the red cells and both anti-A and anti-B antibodies in the plasma.

 

If you are a Type A person, you do not carry antibodies against A markers. But you do have antibodies against Type B blood.

If you are a Type B person, you have antibodies against Type A cells.

If you are a Type O, you have antibodies against both Type A and B!

 

Rh Positive	Population	Rh Negative	Population
A Positive (A+)	34%	A Negative (A-)	6%
O Positive (O+)	38%	O Negative (O-)	7%
B Positive (B+)	9%	B Negative (B-)	2%
AB Positive (AB+)	3%	AB Negative (AB-)	1%