What is the importance of blood groups in transfusion, in terms of agglutination?

Matching blood types are very important in blood transfusions. If the body does not recognize the new blood as is own type, agglutination occurs, which is where the body's antibodies clump together around the new blood to destroy and absorb it. Not only does this neutralize the new blood making it ineffective, but the large clumps can also for blockages in the vascular system. There are 6 main blood types: A, B, O, AB, AO, BO. They can be matches exactly, or with other close types. For example, A can be matched with A, AB, AO (and O). Type O is a universal blood donor, for any blood type can accept it.