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

These days, nearly everyone knows their blood type, and whether it is A, B, AB, or 0, and Rhesus negative or positive. This division of blood into types was first done 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 did this by spinning the blood at high speed in a centrifuge. Then he took the serum and added red cells from different people. They acted in two different ways: either the cells mixed with the serum, or they clumped together (clotted), which is called ‘agglutination’.

Numerous attempts at blood transfusion had been made in the past, but this observation explained for the first time why many had failed. When introduced blood was not of exactly the same type as that in the body, it resulted in the clumping of red cells, and the patient died. Quick tests of blood samples to discover whether agglutination will occur is now made prior to a transfusion being made.

DIVIDING BLOOD INTO GROUPS
Red blood cells contain substances called antigens. These help create antibodies that fight infection and disease. Landsteiner suggested that his experiment showed the presence of two specific antigens, which he labelled 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 each person 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 used in their investigations. 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 an excellent means to uncover crucial evidence in a forensic investigation. If, for example, a victim’s ABO type is 0, and remains of blood of this type are discovered on the clothing of a suspect whose type is A, there is a likelihood that they have come from the victim.

Making use of the many other blood typing systems now available, this probability is greatly increased. If blood of type O occurs in 43% 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 one 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 narrow the group of suspects.

In 1925, another important discovery occurred. Around 80% 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, in particular the white blood cells and those of the lining of the mouth. Blood typing is now 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 experienced 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 result from the pumping action of the heart. A major artery can spray the blood a considerable distance.

Pools form around the body of a bleeding person. If there is more than one pool, he either dragged himself, or was dragged, from one spot 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 if the body was carried, and smears if it was dragged.

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