When Europeans first experimented with blood transfusions in the 17th century, so many patients died that the procedure was outlawed in England, France, and Italy. It is said that the Incas in South America began transfusing blood much earlier and that fewer deaths resulted. If so, the reason, not understood at the time, may have been that nearly all of the Incas belonged to the same blood type, while the Europeans, like most groups of people, belonged to different and incompatible types. Today, blood transfusion is safe only because blood samples from the donor and recipient are tested to ensure that no dangerous transfusion reaction can occur from the mixing of incompatible blood.
             In the ABO system, human blood is classified into four types: A, B, AB, and O. If your blood is type A, your red blood cells carry a protein called Antigen A and your plasma, a protein called antibody b. If you are type B, your blood contains antigen B and antibody a. Blood type AB carries both antigens but no antibody, while type O blood has neither of the antigens but both of the antibodies. These categories are important in transfusion because certain antigens and antibodies are hostile to each other. Shaped so that they can lock together, mutually hostile antigens and antibodies adhere in clumps that can cause fatal blood-vessel blockages. Generally, people with type A blood can safely receive blood from A's and O's, while type B recipients are safe with blood from B's and O's. People whose blood types is AB are known as universal recipients, because their blood is compatible with types AB, A, B and O. Type O people, on the other hand, are safe only with blood from type O donors, but they are themselves, so-called universal donors, because they can give blood to anyone.
             In a routine count the blood's basic components, red cells, hemoglobin, while cells, and platelets, are enumerated from a small sam
             ...