Blood Types: The ABO System

In 1901, Karl Landsteiner reported that blood has "types." By matching these types, blood can be successfully transfused. The donated blood must be compatible with the recipient's blood, and transfusing incompatible blood can result in severe complications -- even death.

Specific proteins called antigens, which are found on the surface of red blood cells, and antibodies, which are found in the plasma, determine the four different types of blood, which are as follows:

1. Type A with A antigens on the red cells and anti-B antibodies in the plasma.
2. Type B with B antigens on the red cells and anti-A antibodies in the plasma.
3. Type AB with both A and B antigens on the red cells and no type antibodies in the plasma.
4. Type O with no type antigens on the red cells and both anti-A and anti-B antibodies in the plasma.

The Rh Antigen

Those who have the Rh antigen on their red cells are typed Rh positive, while those without the Rh antigen are Rh negative. When matching blood, both the ABO system and the Rh types must be considered. For example, if you have both the A antigen and the Rh antigen on your red cells, you are type A+.

Refer to the chart below to determine which red cell types are compatible:

Do You Know Your Blood Type?

The various proportions of blood groups that occur in the average population of the United States are shown below. These percentages vary within different ethnic groups.

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