Blood Types

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Blood Types

A blood type is a certain class or group of blood that has particular properties. There are four major human blood types, which are inherited, and each of which has a characteristic protein on the surface of its red blood cells. Individuals who share the same type of proteins belong to the same blood group or type. It is essential to know a person's blood type before a blood transfusion can be given.

Since the Middle Ages (a.d. 500–1450), doctors thought that if there were only some way to replace the blood a person lost due to injury, they could possibly save lives. Once precision instruments were developed in the seventeenth century that could be used to inject one person's blood into another, blood transfusions were attempted. In far too many of these experiments, however, the results were just the opposite of what was expected. Many patients died and those that did not often became even more ill. Since no one had any idea why blood could not simply be transfused from one person to another, blood transfusions were eventually banned in most of western Europe after the late seventeenth century.


Research on blood did continue, and in the late 1800s, several researchers noted that when blood cells from one animal or person were mixed with cells from another, they stuck together in clumps. This was called agglutination. While studying this clumping phenomenon, the Austrian physician Karl Landsteiner (1868–1943) discovered that not all blood always clumped with other blood. For example, one sample would clump with red cells from person A but not with person B. Another sample might clump red cells from person B but not from person A. Still another sample might clump them both, while yet another might clump neither. Eventually, Landsteiner was able to clearly identify four main blood groups that he named A, B, AB, and O.

Further research showed that these four blood types differed because of the type of protein that was located on the surface of the red blood cells and in the blood's plasma (the fluid part of the blood). These proteins on red blood cells came to be called antigens (a kind of chemical identification tag), while those in plasma were called antibodies (proteins that destroy foreign substances). In what came to be known as the ABO system, there are two antigens (A and B) and four blood groups (A, B, AB, and O). People with type A blood have the A antigen; people with type B blood have the B antigen; type AB people have both; and type O people have neither.

This led Landsteiner to formulate a simple pattern of who can receive what from whom. The first rule is that people in the same blood group can accept blood from each other with no ill effects. Next, blood types A and B are incompatible and cannot receive blood from each other, but they can receive blood from O (since it has no antigens). Blood type AB can accept blood from A or B (since they have both A and B antigens), as well as from O (which has no antigens). The AB blood type is therefore called the "universal recipient." Type O can give blood to all other groups, but can only receive blood from its own type. It is therefore called the "universal donor."


The Rh (rhesus) factor system is another blood group that was discovered by Landsteiner and his associates in 1940. First discovered in rhesus monkeys, it was found that about 85 percent of the human population was Rh positive, meaning that their blood cells carried the D antigen or rhesus antigen. Those that did not were Rh negative. A person's Rh factor becomes important during pregnancy. A fetus (unborn child) that is carried by an Rh-negative woman who developed Rh antibodies by previously carrying an Rh-positive baby can have its red blood cells attacked by these antibodies, resulting in death. The most common blood type in the United States is O+ (O Rh positive). It is found in 38 percent of the population. The type A+ (A Rh positive) is found in 34 percent of the population. Knowing ahead of time a person's Rh factor makes it possible to avoid incompatible transfusions and to correct any incompatibility by a blood transfusion either in the womb or directly after birth. Landsteiner's discovery of blood types has made blood transfusions routine and safe, and has saved the lives of millions of people.


Austrian-American immunologist (a person specializing in the study of the immune system) Karl Landsteiner (1868–1943) discovered the main types of human blood. His blood-typing system made blood transfusions possible and saved countless lives. Awarded the Nobel Prize for Physiology and Medicine in 1930, he also discovered the Rhesus (Rh) blood factor and that polio is caused by a virus (a disease-causing agent).

Since at least the Middle Ages (a.d. 500–1450), doctors had been intrigued by the idea that severe blood loss might be treated simply by injecting the blood of one person into another. Once instruments precise enough to be able to do this were produced in the seventeenth century, blood transfusions were attempted. Sometimes they would work and save a patient, but much more often the transfusion itself would kill the person who was receiving someone else's blood. This happened so often that blood transfusions eventually were banned in most European countries. Until the problem was taken up by Karl Landsteiner, no one knew the reason that one person's blood could not be transferred to anyone else. All anyone knew was summed up by folk wisdom which simply said that everyone's blood was different.

Karl Landsteiner was born in Vienna, Austria, and entered medical school at the age of seventeen. By the time he was twenty-three, he had received his doctorate in medicine and went to work in the field of organic chemistry, studying with some of the best chemists in Europe. By around 1896, he became interested in the nature of antibodies, which are special proteins that circulate in the blood and lock on and disable any foreign substance that enters the body. By 1900, he was studying how blood agglutinates, or clumps, together when it is brought into contact with the blood of another person. No one could properly explain why this happened, but Landsteiner believed that it was due to something unique in the blood of each individual. He then began a series of experiments that showed that there were often very different things going on when blood clumped together. For example, the blood of one person might clump the blood from person A but not from person B, while another sample might clump blood from person B but not from person A. Another might clump both, and yet another might clump neither. Instead of giving up in the face of what seemed chaos, Landsteiner kept at his experiments and data-gathering and eventually saw that a real pattern existed in all of this. From his observations he came up with the idea of mutually incompatible blood groups, which he finally was able to sort out into four groups he called A, B, O, and AB. Landsteiner explained that blood contained certain antibodies that triggered a clumping reaction when one group, or type of blood, was mixed with another. He then showed that blood transfusions were possible if blood was "typed" and if the right type of blood was given to the right patient. Guided by Landsteiner's work, the first successful blood transfusions were achieved at Mt. Sinai Hospital in New York in 1907. Thereafter, Landsteiner's achievement saved many lives on the battlefields of World War I (1914–18), where transfusions of "compatible" blood were first performed on a large scale.

Although Landsteiner continued to work on antibodies, he turned his attention to studying the disease called polio (a viral disease that attacks the nervous system) and was able to show that it was not caused by a bacteria, but was instead traceable to a virus. In the 1920s, Landsteiner joined the Rockefeller Institute for Medical Research in New York and became an American citizen. Although officially retired by 1939, he kept working and, in 1940, discovered yet another blood factor that came to be called the Rh factor (named after the Rhesus monkeys in which it was first discovered). The Rh factor was shown to be responsible for a disease that occurred when mother and fetus have incompatible blood types and the fetus is injured or killed by the mother's antibodies. Landsteiner's brilliant work on blood groups has had a major impact on medicine and health, making life-saving blood transfusions possible. His work on blood typing also is regularly applied in legal and criminal cases in which blood is used as evidence. Landsteiner never really stopped working and died after suffering a heart attack at his laboratory bench.

[See alsoBlood; Rh Factor ]