At the scene of a violent crime, the examining officer is likely to discover blood and evidence of other bodily fluids. These are able to tell a lot about what happened, not only regarding how the crime was committed, but also about the persons involved.
These days, nearly everybody knows their basic blood type, 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, called the 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 acted in two different ways: either the cells mixed with the serum, or they clumped together (clotted), which is called ‘agglutination’.
A number of 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 the same type as that in the body, it resulted in clotting, and the patient died. 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 carry substances called antigens. Antigens help create antibodies that fight infection and disease. Landsteiner thought that his experiment showed the presence of two specific antigens, which he labeled A and B. The discovery of these antigens enabled 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 a 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 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 a powerful tool for uncovering 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 likelihood that they have come from the victim.
Making use of the many other blood typing systems now available, this probability can be 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 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 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 valuable discovery occurred. Around 80% of humans are ‘secretors’. This means their saliva, urine, perspiration, and semen contain the same substances as their blood, and are able to be used for typing in much the same way. In 1940, two British researchers discovered it was possible to distinguish between female and male body cells, especially 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 experienced 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 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 blood a considerable distance.
Pools form around the body of the bleeding person. If there is more than one pool, he either dragged himself, or was dragged, from one spot to another before dying. Smears are likely also 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.