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At the scene of a violent crime, the examining officer will likely discover blood and traces of other bodily fluids. These are able to tell a great deal about what happened, not only about details of how the crime was committed, but also about the persons involved.

These days, nearly everyone knows their blood type, and whether it is A, B, AB, or 0, and Rhesus negative or positive. This categorising of blood into types was first made by Austrian physiologist Karl Landsteiner at the end of the 19th century. In his experiments, he took samples of blood and separated the red cells from the liquid, which is called serum. He achieved this by spinning the blood at high speed in a centrifuge. Then he took the serum and added red cells from different people. They behaved in two different ways: either the cells mixed with the serum, or they clumped together (clotted), (”agglutinated”).

Numerous attempts at blood transfusion had been made in the past, but this observation explained for the first time why a great proportion had failed. If introduced blood was not of the same type as that in the body, it produced clotting, and the patient died. Tests of blood samples to discover whether agglutination will happen is now made before a transfusion is performed.

DIVIDING BLOOD INTO GROUPS
Red blood cells contain substances called antigens. These help create antibodies that fight infection and disease. Landsteiner believed that his experiment showed the presence of two specific antigens, which he labeled A and B. The discovery of these antigens caused him to divide human blood into 4 basic groups:

Group A: antigen A present; antigen B absent
Group B: antigen A absent; antigen B present
Group AB: both antigens A and B present
Group 0: both antigens absent

The specific blood group of an individual depends on the genetic inheritance from both parents. Known as ABO typing, it has been used, for example, to help identify the biological father in paternity cases. How common each group is can vary from one national population to another. In the United States, for example, the relative proportions of ABO groups are roughly 39 percent A, 13 percent B, 43 percent 0, and 5 percent AB.

In 1927, Landsteiner found two other antigen types, labeling their occurrence as M, N, and MN. In 1940, working in the United States, he and A.S. Wiener discovered the Rhesus factor, named after the Rhesus monkeys they investigated. Since then, other researchers have introduced more than a dozen further group systems. Different proteins and enzymes associated with specific blood groups have also been identified.

WHAT THIS MEANS FOR FORENSICS
The ability to identify blood type is a powerful means for revealing important evidence in a forensic investigation. If, for example, a victim’s ABO type is 0, and bloodstains of this type are discovered on the clothing of a suspect whose type is A, there is a possibility that they have come from the victim.

Making use of the many other blood typing systems now available, this probability is increased greatly. If blood of type O occurs in 43 percent of the population, the substance haptoglobin-2 in 36% of these, and the enzyme PGM-2 in 5%, then the probability of an individual having these three blood types together is 43 x 36 x 5 = 7,740 in 1 million. In other words, around eight people in every thousand will have this specific type of blood. It is still insufficient to obtain a conviction on this evidence alone, but it can help to reduce the number of suspects.

In 1925, another valuable discovery occurred. Around 80 percent of humans are ’secretors’. This means their saliva, urine, perspiration, and semen contain the same substances as their blood, and can be used for typing in a similar way. In 1940, two British researchers discovered that it was possible to distinguish between female and male body cells, particularly the white blood cells and those of the lining of the mouth. Blood typing has now become so precise that recently one scientist showed that he could distinguish between the blood of his twin daughters, who were genetically identical, because one had had chicken pox and the other had not.

SPLASHES OF BLOOD
At the scene of a violent homicidal attack, blood may be present in considerable quantities. Not only will it be found on the victim, but also on the weapon and the surroundings. Indoors, the floors, walls, and even the ceilings may be splashed. Careful observation of these bloodstains can provide valuable clues about what took place. Bloodstains and splashes are classified into six basic types.

Round drops are found on horizontal surfaces; depending on the height from which they fell, they can spray out into a starlike shape. Splashes of blood are shaped like an exclamation mark; they show that blood has flown through the air and hit a surface at an angle. While a victim is still alive, spurts of blood come from the pumping action of the heart. A major artery can spray blood a great distance.

Pools of blood form around the body of a bleeding victim. If there is more than one pool, he either dragged himself, or was dragged, from one area to another before dying. Smears will also be found if this happens. Trails are left when a bloody corpse is moved. There will be drops found if the body was carried, and smears if it was dragged.

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