In 1898, it was discovered that inherited differences in people's red cells were the cause of many of the incompatibilities seen with transfusions. Four blood types were identified. During World War I, when human blood was needed for transfusions for wounded soldiers, studies of how to preserve and transport blood began.
Not until World War II, however, did the development of effective preservative solutions make blood transfusions widely and safely available. There have been many advances since then, including the discovery of additional types of blood such as the Rh positive and Rh negative classifications.
Today, thanks to these advances, full utilization is made of nearly every blood donation. Elements of blood can be separated by centrifuge. Plasma can be preserved by freezing. Each blood element can be used to treat different diseases.
Blood is now tested for diseases it may carry, and any blood testing positive for a disease is destroyed.