Type "O" can be used for emergency transfusions, since 85 percent of the population can safely receive "O" positive red blood cells. Because type "O" is also the most prevalent blood type (45 percent), it is used most frequently for routine transfusions.
Type "A" blood is used for routine transfusions to type "A" patients, who have the next most prevalent (40 percent) blood type. Six percent of the population has "A" negative blood type. The plasma from "A" blood is particularly rich in clotting factors used to treat bleeding disorders such as hemophilia.
Type "AB" red blood cells are used by very few patients (about 4 percent). However, the plasma from type "AB" donors is very valuable because it can be given safely to any patient for emergency transfusion needs, regardless of their blood type. Type "AB" plasma is also used for transfusions to newborn babies; a single donation may be divided among several infants.
Type "B" blood is needed for routine transfusions to type "B" patients, who represent 11 percent of the population.