n A
classification of blood that is based on the presence or absence of
antigen|antigens in the
red blood cell|red blood cells of an individual. For the purposes of
blood transfusion three antigens referred to as "A", "B" and "RhD" (out of a total of 29) are the most important. Based on their presence the human blood can be classified to
A,
AB,
B, and
O 'blood types' which are further divided to
Rh positive or negative.