Everyone’s blood isn’t exactly the same, but this important fact was not known to scientists until 1900.
Before then, blood transfusions were given from person to person without any thought to blood types. When it was discovered that in about half the cases, the patient got worse after a transfusion, and sometimes even died, scientists began to study why.
They learned that blood could be grouped, or typed, according to the presence or absence of a substance called antigens on the outside of the red blood cells.
These antigens, if mixed with other “foreign” antigens, can cause the blood to clump, or stick together. This destroys the red blood cells, blocks the small blood vessels, and can cause serious illness or death.
But scientists also discovered that some blood types can be safely mixed without this harmful result.
Therefore, when blood transfusions need to be given, hospitals today first perform a cross match, a mixing of some of the patient’s blood with that of a donor, to be certain that the two types won’t clump.
The four different blood types have been named type 0, A, B, and AB.
Type 0 blood has been found to mix safely with all the other types and has been called the “Universal Donor”.