Serology: It's in the Blood
The analysis of the properties and effects of serums (blood, semen, saliva, sweat, or fecal matter) is called serology. We'll concentrate here on the principal tests used to identify blood. According to Henry C. Lee, a forensics expert who has assisted law enforcement in over 6,000 major criminal investigations — including that of O. J. Simpson — blood evidence is found most often in "crimes of violence such as homicide, assault, and sexual assault." It may be in the form of fresh liquid, coagulated, dried, or as a small drop or stain, and each form involves a different method of preservation and collection.
We all have about ten pints of blood getting pumped throughout our bodies. When wounded, bodies leak or spray blood, and the behavior of blood in flight tends to be unaffected by such things as temperature, humidity, or atmospheric pressure. In other words, it's uniform.
Despite how well the crime scene may get cleaned up, even the finest trace of blood can often be detected and further tested. It is often the case that while the perpetrator may scrub down the obvious places, he can still miss between floorboards, under pipes, and inside drains. Merely by pouring water on some tiles at a murder scene and pulling them up wherever the water flowed beneath them, one detective found the only existing trace of the crime — blood. His discovery so surprised the killer, who felt certain he'd done a through job of cleaning up, that he instantly confessed.
Different blood types were recognized in 1875, but it wasn't until 1901 that Karl Landsteiner named and standardized the groups. Red blood cells carry a substance called an antigen, which produces antibodies to fight infection, and there appeared to Landsteiner to be several different types. In a centrifuge, he separated the red blood cells from the plasma or watery serum in which they are carried through the body. Then adding red blood cells from various other subjects, he found two distinct reactions — clumping and repelling. He labeled them types A (antigen A present, anti-B antibody present, but antigen B absent) and B (antigen B present, antigen A absent). Then a third reaction was labeled C (both antigens A and B absent), but was relabeled later as O. Then another type of serum was discovered, and this fourth type was labeled AB (both antigens present). It soon became clear that the blood type depended on genetic inheritance from parents, which helped with paternity tests. Type A and O are the most common in the human population, with AB the most rare.
Then Dr. Leon Lattes in Italy developed a procedure to apply blood testing to stains on fabric and other materials. He did this by finding a way to use saline solution to restore dried blood to its liquid form, although in 1932, he invented a way to test for antibodies in dried blood flakes as well.
In 1940, Landsteiner also discovered the rhesus factor in blood, labeling it Rh+ if the antigen was present in the red blood cells and Rh- if not. Today, blood typing also includes different types of enzymes and proteins that perform specific activities in the body, which helps to individualize the blood. (More than 150 serum proteins and 250 cellular enzymes have been isolated, as well as many more antigens.)
Nine years went by before British scientists came to the conclusion that the nuclei of female blood cells contain a chromosome-related structure that set them apart from those of males, and they named this the Barr Body. It added one more dimension to identification via blood samples.
When a darkish substance is found at a crime scene, it must first be determined to be blood. There are several tests — presumptive tests used strictly for screening — that will differentiate between blood and other substances, but if other chemicals are present at the scene to which the test chemicals are sensitive, the tests may be vulnerable to corruption. For that reason, these tests are done with great care. A positive result from any of them is an indication to go ahead and use other tests to confirm.
Before doing anything, the crime scene investigators must take some precautions in order to avoid both biohazard to themselves and sample corruption. In his Physical Evidence in Forensic Science, Henry Lee suggests the following:
- Wear latex gloves, surgical masks, and full coverage gowns
- Eye-coverings are needed for collecting liquid samples
- Keep hands out of areas that are hidden
- Label all blood samples
- Package dry samples in bags, as well as stained clothing
- Add a note of precaution if biohazards like AIDS or hepatitis are suspected
- Decontaminate all nondisposable items
- Destroy tags, forms, or reports splashed with blood
- Clean up hands with diluted bleach, and dispose of contaminated clothing
The first test is simply the use of a powerful light moved across every surface of a crime scene. That yields possible traces for visual inspection.
If nothing is seen, but there is reason to suspect blood had been present, a chemical called luminol is sprayed across the scene because it reacts to blood by making it luminescent. It only takes about five seconds. The procedure requires that the room be considerably darkened in order to see the faint bluish glow, and the intensity of the glow increases proportionately to the amount of blood present. It works even with old blood or diluted stains, and can illuminate smear marks where blood has been wiped away. However, there is one problem with this test: luminol can destroy the properties of the blood that investigators need for further testing. Its use is limited to proving that blood is present even if not visible.
The Kastle-Meyer Color Test uses a solution of phenolphthalein and hydrogen peroxide on a piece of filter paper, and when blood of any quantity is present, it turns pink. However, it also turns pink in the presence of potatoes or horseradish, so care must be taken at the scene.
Sometimes microcrystalline tests are also performed. The two most often used are the Takayama and Teichmann tests. Both add specific chemicals to the blood to make it form crystals with hemoglobin derivatives. These tests are also sensitive to other materials that may be present in a bloodstain.
From there, investigators use the precipitin test to determine whether the blood is of animal or human origin. German biologist Paul Uhlenhuth discovered that if he injected protein from a chicken egg into a rabbit, and then mixed serum from the rabbit with egg white, the egg proteins separated from the liquid to form a cloudy substance known as precipitin. In other words, it forms an antibody. In the forensic test for human blood, either a sample of the suspect blood is put into a test tube over the rabbit serum or it's used in the "gel diffusion" test, where it's placed in gel on a glass slide next to a sample of the reagent (anti-human serum). Passing an electric current through the glass, the protein molecules filter into the gelatin and toward each other. If a line forms where they meet — called a precipitin line — that means the sample is human blood.
After that, analysts can go ahead and determine blood type with an ABO test, and then work on the gender of the person from whom the blood came. To get a more thorough enzyme/protein profile, they use electrophoresis (a blood-soaked piece of cotton placed in gelatin on a slide and submitted to electric current).
In 1925, another blood-related discovery important to criminal investigation was made. Around 80 percent of the human population were found to be "secretors," which means that the specific types of antigens, proteins, antibodies, and enzyme characteristic of their blood can be found in other bodily fluids and tissues. In the case of a secretor, investigators can tell the blood type by examining the saliva, teardrops, skin tissue, urine, or semen. In a rape case, for example, where the perpetrator is a secretor, potential suspects can be narrowed down through blood type analysis.
These days, thanks to discoveries in 1985, DNA technology has replaced the tests for specific enzymes and proteins. It's more accurate to match DNA from a blood sample at a crime scene to a source than to draw up an entire blood profile.
Yet blood at a crime scene can offer even more clues than gender and type. Aside from the mere presence of blood, the different ways that blood lands on a surface has given rise to a forensic subspecialty known as blood-pattern analysis, or BPA. Let's hear about that from one of the experts.