Survival of Cells. Blood type refers to thousands of proteins (antigens) on surface of cell membranes, as recognized by antibody proteins. Abo and rh types were the first 2 & most important (in terms of transfusion reactions) characterized historically, there are hundreds more. Antibodies in the recipient plasma attack donated cells & antibodies in the donor plasma attack recipient’s cells > transfusion reaction.
Hemocoagulation. Giving the wrong blood type to someone can result in destruction of the new blood, a simplified rejection. A recipient b blood type has antibodies against a so if a blood is transfused into the patient, the anti-a will destroy the new a marked blood cells.