Why does a human body accept or reject donated blood?

The blood types of humans are broken up into 4 ABO groups and further by Rh (Positive or Negative). Therefore, there are a total of 8 Blood groups: O positive, O negative, A positive, A negative, B positive, B negative, and AB positive, and AB negative. The A and B refer to antigen or "receptors" on the red cells. Therefore, O red cells have neither A nor B antigens on their red cells, and can be given to everyone, while AB red cells have both A and B antigens, and can only be given to AB people. The reason for this is that every person has antibodies to the opposing red cell other than their own. Thus, an A type person has B antibodies, while a B type person has anti-A antibodies, and an O person has anti-A and anti-B antibodies. An AB type person can receive all types of blood. However, a compatibility test must be performed prior to transfusion, known as a crossmatch, using the recipient's serum (with antibodies) against the donor unit's cells (the antigens). If no reaction takes place, the cells are compatible with the patient and can be transfused. If there is a reaction (agglutination, or clumping of the red cells in the patient's serum) then the unit it not compatible and should not be transfused.