There are a total of eight blood groups in humans, determined by the presence or absence of antigens and antibodies. The groups are type O positive and negative, type A positive and negative, type B positive and negative, and type AB positive and negative.
The primary classification of blood groups is dependent on the presence of type A and B antigents. Type O blood has neither, type A and B blood contain only those specific antigens, and type AB blood contains both. In addition, a third antigen, the Rh factor, determines whether the blood is considered positive or negative. In addition to the antigens present in the red blood cells, each type of blood has antibodies in the plasma that work against the antigens not present. Type A blood has anti-B antibodies, and vice versa.
The presence of these antigens and antibodies determines which type of blood may be given to any recipient. If a patient is given blood containing antigens for which he has antibodies, a life-threatening reaction can occur. This is why O is considered the universal donor type, since it contains no antigens. However, only O-type blood can be given to O blood group patients, since O blood contains both anti-A and anti-B antibodies.