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

Nearly everyone knows their blood type, whether it is A, B, AB, or 0, and Rhesus negative or positive. This categorising 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, 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 responded in two different ways: either the cells mixed with the serum, or they clumped together (clotted), which is called ‘agglutination’.

Many attempts at blood transfusion had been made in the past, but this observation explained for the first time why many had failed. If introduced blood was not of exactly the same type as that in the body, it resulted in agglutination, and the patient died. Quick 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 carry substances called antigens. These help make antibodies which 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 enabled him to divide human blood into four 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 particular blood group of each person depends on the genetic inheritance from both parents. Known as ABO typing, it has been used, for example, to identify the biological father in a paternity case. 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 discovered two other antigen types, labelling 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 additional 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 tool to uncover crucial evidence in a forensic investigation. If, for example, a victim’s ABO type is 0, and bloodstains of this type are found on the clothing of a suspect whose type is A, there is a likely probability 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 percent of these, and the enzyme PGM-2 in five percent, then the probability of an individual having these three blood types together is 43 x 36 x 5 = 7,740 in 1,000,000. In other words, around 8 people in every 1,000 will have this specific type of blood. It is still not enough to obtain a conviction on this evidence alone, but it can help to reduce the group of suspects.

In 1925, another valuable discovery occurred. Around 80% of people 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 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 had chicken pox and the other hadn’t.

SPLASHES OF BLOOD
At the scene of a violent homicidal attack, blood may be present in great quantities. Not only will it be 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 of blood form around the body of a bleeding person. 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 body is moved. There will be drops if the body was carried, and smears if it was dragged.

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