According to Britannica.com, the business of classifying blood into various groups or types was first described by Karl Landsteiner in 1901. More specifically, Landsteiner, an Austrian-born U.S. immunologist, discovered the "ABO" system, made up of four groups (A, B, AB, and O) into which all blood can be classified. Apparently it's the presence or absence of certain "antigens" in your blood that determines type.

But that's just the tip of the iceberg. Further research by Landsteiner and others lead to the discovery of 14 main types of blood group systems. Landsteiner also participated in the discovery of the Rhesus blood factors and the polio virus.

You can read more about blood grouping at the WebMD Medical Encyclopedia entry on the subject. To learn how blood types pertain to you, try the Yahoo! Health section on blood typing. Lastly, for more on Karl Landsteiner, read the biographical sketch available on the Nobel Foundation's web site (he won the prize in 1930).

Experiments with blood transfusions, the transfer of blood or blood components into a person's blood stream, have been carried out for hundreds of years. Many patients have died and it was not until 1901, when the Austrian Karl Landsteiner discovered human blood groups, that blood transfusions became safer.

Mixing blood from two individuals can lead to blood clumping or agglutination. The clumped red cells can crack and cause toxic reactions. This can have fatal consequences. Karl Landsteiner discovered that blood clumping was an immunological reaction which occurs when the receiver of a blood transfusion has antibodies against the donor blood cells.

Karl Landsteiner's work made it possible to determine blood groups and thus paved the way for blood transfusions to be carried out safely. For this discovery he was awarded the Nobel Prize in Physiology or Medicine in 1930.

What is blood made up of?

An adult human has about 4–6 liters of blood circulating in the body. Among other things, blood transports oxygen to various parts of the body.

Blood consists of several types of cells floating around in a fluid called plasma.

The red blood cells contain hemoglobin, a protein that binds oxygen. Red blood cells transport oxygen to, and remove carbon dioxide from, the body tissues.

The white blood cells fight infection.

The platelets help the blood to clot, if you get a wound for example.

The plasma contains salts and various kinds of proteins.

What are the different blood groups?

The differences in human blood are due to the presence or absence of certain protein molecules called antigens and antibodies. The antigens are located on the surface of the red blood cells and the antibodies are in the blood plasma. Individuals have different types and combinations of these molecules. The blood group you belong to depends on what you have inherited from your parents.

There are more than 20 genetically determined blood group systems known today, but the AB0 and Rh systems are the most important ones used for blood transfusions. Not all blood groups are compatible with each other. Mixing incompatible blood groups leads to blood clumping or agglutination, which is dangerous for individuals.

Nobel Laureate Karl Landsteiner was involved in the discovery of both the AB0 and Rh blood groups.

AB0 blood grouping system

According to the AB0 blood group system there are four different kinds of blood groups: A, B, AB or 0 (null).

 Blood group A

If you belong to the blood group A, you have A antigens on the surface of your red blood cells and B antibodies in your blood plasma.

 Blood group B

If you belong to the blood group B, you have B antigens on the surface of your red blood cells and A antibodies in your blood plasma Blood group AB

If you belong to the blood group AB, you have both A and B antigens on the surface of your red blood cells and no A or B antibodies at all in your blood plasma.

Blood group 0

If you belong to the blood group 0 (null), you have neither A or B antigens on the surface of your red blood cells but you have both A and B antibodies in your blood plasma.

Rh factor blood grouping system

Many people also have a so called Rh factor on the red blood cell's surface. This is also an antigen and those who have it are called Rh+. Those who haven't are called Rh-. A person with Rh- blood does not have Rh antibodies naturally in the blood plasma (as one can have A or B antibodies, for instance). But a person with Rh- blood can develop Rh antibodies in the blood plasma if he or she receives blood from a person with Rh+ blood, whose Rh antigens can trigger the production of Rh antibodies. A person with Rh+ blood can receive blood from a person with Rh- blood without any problems.