Determination of the type and characteristics of blood, blood testing, bloodstain examination, and preparation of testimony or presentations at trial are the main job functions of a forensic serologist, who also analyzes semen, saliva, other body fluids and may or may not be involved with DNA typing. It must be recognized, however, that in many crime labs, there may be no clear distinction between job title and job function.
A particular laboratory may not have a serologist on staff, their functions being performed by a criminalist, a biochemist, a forensic biologist, or other technician. Such personnel would normally possess a Bachelor's or Master's degree, while a chief serologist would possess an M.D. or Ph.D. It's rare to find chief serologists, and the Bachelor's degree seems common. A few states have laws which make serological examinations admissible by statute without the necessity for testimony by an expert, the purpose of which is to insulate and protect their crime lab technicians.
Other states rely upon their Chief Medical Examiner's office, forensic pathologists, or board-certified toxicologists. Professors of biochemistry, hematology, and immunology are often "borrowed" as experts by both prosecution and defense. In certain specialized areas involving bloodstain examination (such as blood spatter analysis), courts will ordinarily qualify someone as an expert who has no formal education but specialized training and has conducted a sufficient number of examinations and accumulated enough reference patterns to be able to demonstrate the basis of their opinion.
These kinds of experts are usually law enforcement personnel, and their testimony is most frequently found in those states which have modified Frye or embraced Coppolino. Further, blood and bloodstain evidence is such an integral part of most crime scenes that a police investigator/bloodstain specialist might be found, in some jurisdictions, testifying on the ultimate issue, even though this usurps the province of the jury. Federal Rule of Evidence 704 allows this to some degree. The Daubert impact has brought conditions more in line with Federal Rule of Evidence 702 than with a statistical showing of validity and reliability. Probability estimates are frequently used in blood testimony.
There's no substitute for it, whether for medical or forensic purposes. Its presence always links suspect and victim to one another and the scene of violence. Bloodstain patterns tell a lot about position and movement during the crime, who struck whom first, in what manner, and how many times.
This destroys most alibi and self-defense arguments for crime, and at the very least, trips most suspects up in their explanation of what happened. Over the years, criminals have tried many ingenious ways to hide, clean up, and remove blood evidence, but it's an area where criminal justice technology has always stayed one step ahead of them. Blood is a slightly alkaline fluid made up of water, cells, enzymes, proteins, and inorganic substances that circulate throughout the vascular system carrying nourishment and transporting oxygen and waste.
The most fluid portion of blood consists of plasma, which is mostly water, and serum, which is yellowish and contains white cells and platelets. The most non-fluid portion of blood consists of red cells which outnumber white cells by five hundred to one. While medical scientists are more interested in white cells, forensic scientists are more interested in red cells and secondly with serum. With serum, the analyst can determine the freshness of a blood sample because serum clots several minutes after exposure to air (a centrifuge is necessary to separate clotted material from the rest of serum).
In serum are also found antibodies, which have important forensic implications. With red cells, the analyst looks for smaller substances residing on their surfaces, such as antigens, which have important forensic implications. One might even say that forensic serology is all about antigens and antibodies, but that is the domain of immunology.
In forensic law, blood has always been considered class evidence, but the potential exists for individualized blood typing, and even today, forensic serologists can provide testimony with some strong probability estimates linking a single individual, and that individual only, to a bloodstain. Consider that identical twins may have the same DNA profile but completely different antibody profiles, and you begin to see how promising the field of forensic serology really is.
The typing of blood, with what is now called the A-B-O system, was discovered in 1901. A few years later, starting around 1937, a series of antigen-antibody reactions were discovered in blood, the most common ones being ABH, MN, Rh, and Gm (over 100 antigens exist). Most people are only familiar with the Rh factor, which is technically the D antigen. There are more than 256 antigens, and 23 blood group systems based on association with these antigens.
A basic principle of serology is that for every antigen, there exists a specific antibody. For routine blood typing, all you need are two antiserums: anti-A and anti-B, both easily available commercially. By dripping a droplet of these antiserums in samples of blood, you see which samples maintain a normal appearance (at about 200x magnification) and which samples become clotted, or agglutinated.
Blood of type A will be agglutinated by anti-A serum; blood of type B will be agglutinated by anti-B serum; AB blood by both; and O blood by neither. You are essentially determining blood type by injecting the worst possible poison into someone's blood sample to see what happens.
The "O" type is most common among indigenous people (like Aborigines and Native Americans) and Latin Americans. The "A" type is most common among Caucasians and those of European descent. The "B" type is most common among African-Americans and certain Asians (e.g. Thai). The "AB" type is most common among the Japanese and certain Asians (e.g. Chinese).
An interesting phenomenon is that Middle Easterners are somewhat likely to have nucleated red blood cells, whereas normally, red blood cells contain no nucleus. Men generally have more red blood cells than women. Red blood cells are originally formed from stem cells, and stem cells are found in bone marrow, the ribs, breastbone, pelvis, and vertebrae, but red cell production is controlled by a hormone released by the kidney, which in turn, instructs the bones to release more red blood cells.
Rare blood types exist in addition to the basic ABO system. A far more useful breakdown involves the Rh (Rhesus disease) factor. If a person has a positive Rh factor, this means that their blood contains a protein that is also found in Rhesus monkeys. Most people (about 85%) have a positive Rh factor, and doctors are trained to monitor closely any woman who is Rh negative and becomes pregnant.
The Rh system is actually much more complicated than the ABO system because there are about 30 combinations possible, but for the sake of simplicity, Rh is usually expressed as either positive or negative. The Rh factor, like other antigens, is found on the covering of red blood cells. It's common for a forensic scientist to take the percentage distribution of the Rh component, which is expressed as plus or minus.
Subgrouping is also possible under the ABO system. Various extracts can be obtained from plants and seeds to create antiserums that clot type O blood, for example, somewhat selectively. Most major blood groups have at least two major subgroups; O1, O2, A1, A2, etc.
The most commonly used types of antiserums used for this purpose are called lectins. The possibility of individualized blood types is based on the typing of proteins and enzymes. Forensic serologists almost always do this level of typing. Blood proteins and enzymes have the characteristic of being polymorphisms or iso-enzymes, which means they exist in several forms and variants, so each one of them have subtypes. Most people are familiar with at least one common polymorphism in blood: Hb, which causes sickle-cell anemia.
Wet blood has more value than dried blood because more tests can be run. For example, alcohol and drug content can be determined from wet blood only. Blood begins to dry after 3-5 minutes of exposure to air. As it dries, it changes color towards brown and black. Blood at the crime scene can be in the form of pools, drops, smears, or crusts.
Pools of blood obviously have more evidentiary value in obtaining a wet sample. Drops of blood tell the height and angle from which the blood fell. The forensic science of blood spatter analysis says that blood which fell perpendicular to the floor from a distance of 0-2 feet would make a circular drop with slightly frayed edges. Drops from a higher distance would have more pronounced tendrils fraying off the edges (a sunburst pattern).
A blood smear on the wall or floor tells the direction of force of the blow. The direction of force is always in the direction towards the tail, or smaller end, of the smear, or splatter. In other words, the largest area of the smear is the point of origin (a wave cast-off pattern). Blood crusts need to be tested with crystalline methods to make sure it's blood. Refrigerated red blood cells have a shelf life of about 42 days, and the serum containing white blood cells can be refrigerated much longer, almost up to a year.
DNA can be extracted from blood (if white blood cells which always contain a nucleus are present), and also from sperm, bone marrow, tooth pulp, and hair roots. Blood, however, is commonly used in DNA testing, as per the following steps:
1. Blood samples are collected from the victim, defendant, and crime scene
2. White blood cells are separated from red blood cells
3. DNA is extracted from the nuclei of white blood cells
4. A restrictive enzyme is used to cut fragments of the DNA strand
5. DNA fragments are put into a bed of gel with electrodes at either end
6. Electric current sorts DNA fragments by length
7. An absorbent blotter soaks up the imprint; it is radioactively treated, and an X-ray photograph (called an autoradiograph) is produced.