A, B, and O are the three major blood types. The types are defined by molecules on the surface of the blood cells. In people of different blood types, these molecules act as antigens -- immune-response triggers. Each person has a combination of two of these surface molecules. Type O refers to a lack of any molecule. So, the resulting types are type A (AA or AO molecules), type B (BB or BO molecules), type AB, or type O. People of a given type form antibodies against other types. When exposed to another type of blood, a breakdown reaction occurs. This is most relevant when a patient needs a blood transfusion or transplant. The blood types must be matched to avoid a reaction based on ABO incompatibility. For example, a patient with type A blood will react against type B or type AB blood. Similarly, a patient with type B blood will react against type A or type AB blood, and patients with type O blood will react against type A, type B, or type AB blood. Because type O signifies a lack of any surface molecules, type O blood does not cause an immune response. This is why type O blood cells can be given to patients of any blood type, and people with type O blood are called "universal donors." However, people with type O can only receive type O. Since antibodies are present in the blood plasma -- the liquid portion of the blood -- plasma transfusions as well as whole blood transfusions must be matched to avoid causing an immune reaction.
| 
