Not your type?

The four basic blood groups or types, in order of frequency from most common to rarest, are O, A, B, and AB. Blood type is determined by "alleles," or possible types of a gene, that we inherit from our parents.

The different blood types reflect the possible combinations of protein molecules called antigens, which are found on the surface of the red blood cells, and antibodies, which are in the plasma.

Just as when a disease invades the body, antibodies in the blood will attack certain antigens. This means that not all human blood is compatible.

If someone were to be given a transfusion with blood that contains antibodies that are hostile to antigens in his own blood, for example, he might die from a reaction that causes red blood cells to "clump," clogging blood vessels, or to "crack," leaking hemoglobin into the body with toxic effects.

O-negative blood lacks antigens, so people with O-negative blood have been considered "universal donors," whose blood would work harmlessly in anyone's body. It turns out that even some O-negative blood can react with some rare blood types, so the concept of the "universal donor" is now a conditional one, even though O-negative blood will still be given in an emergency if a patient's blood type is not known.

Conversely, people with AB-positive blood in general can receive any type of blood because that type does not contain antibodies that attack the A or the B antigens. (Type O blood lacks those antigens.)

The "plus" and "minus" in blood types refers to a particular antigen called "the Rh factor." Anyone can receive blood without the Rh factor, but only people with the Rh factor can safely receive blood that contains it.

If a pregnant woman needs a blood transfusion during or after labor — rare but possible — she will receive only blood that is compatible with her own — ideally her own specific blood type.

Not all blood is the same

In 1900, the Austrian chemist, botanist and medical researcher Karl Landsteiner realized that not all human blood is alike, that some people's blood contains substances that are toxic to other people's blood.

That began to solve the mystery of why some people who received blood transfusions were fine, while others became ill and often died.

Karl Landsteiner
Karl Landsteiner

Landsteiner subsequently discovered three of the four genetically determined blood groups or types, O, A and B. A couple of years later, Alfred von Decastello and Adriano Sturli, Landsteiner's colleagues in Vienna, identified a fourth blood group, AB. While about 30 blood types have been discovered, the original four essentially cover everyone.

In 1910, at the Heidelberg Institute for Experimental Cancer Research in Germany, Ludwig Hirszfeld and Emil von Dungern demonstrated that blood type is an inherited trait.

In the speech he made when he accepted the Nobel Prize in 1930 for his work, Landsteiner described the mystery blood presented, and how he and his fellow researchers unraveled its secrets.

In 1922, Landsteiner moved to the Rockefeller Institute of Medical Research in New York, where he discovered an extremely powerful blood antigen he called "the Rh factor."

Today, hospital personnel make sure they know a mother's blood type in case she needs a transfusion. She will also be tested for her Rh factor because it can pose a danger to her baby's well being.