|MadSci Network: Immunology|
You are correct, people with type O blood have antibodies to group A and group B antigens. If a type O person is transfused with type A blood, the antibodies present in the type O person's blood will bind to the erythrocytes (red blood cells) in the transfused blood. This will lead to a hemolytic depletion of the transfused blood. Mismatched blood transfusion can also lead to renal damage (due to attempted clearance of the erythrocyte degradation products) and loss of circulatory stability.
So, in short, a person who has type O blood **will** "attack" type A blood, if they are unlucky enough to be transfused with it. This is why hospitals always type match and cross, to make sure that the blood they transfuse will not cause problems in the recipient. This is also why, in the absence of blood type screening, the default is to use Type O Rh- blood, as this is least likely to cause problems (and also why O Rh negative people are referred to as "universal donors" and type AB Rh positive people are referred to as "universal recipients" as theoretically, they could be transfused with any blood). There are more blood groups than A, B and O and Rh+/-, but these are the major ones.
Hope that helps-
Babior and Stossel Hematology:A Pathophysiological Approach, 3rd Ed1994. New York: Churchill Livingstone Inc.
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