Charles graduated from college in 1926 and in 1928, he went to McGill University in Montreal, Canada to study medicine. In 1933, he graduated second in his class of 127 students with a Master of Surgery and Doctor of Medicine degrees. He returned to the U.S. and was an instructor at Howard University.
At Columbia University Presbyterian Hospital, Charles became interested in blood transfusions. He wrote his dissertation on "banked blood" and found that by separating the plasma or liquid part of blood from the whole blood, blood could be combined for up to a week for blood transfusions. He discovered that although everyone has a certain blood type, either type A, B, AB, or O, and cannot receive blood that is not a match, everyone has the same type of plasma.
Did you know that the most common blood type in the world is Type O positive. The most rare blood type is AB negative, and is found in less than 1% of the population worldwide.