Antibody (World of Forensic Science)
Among the many techniques used in forensic science are those that involve the specific immunological recognition of a protein (the antigen). The protein molecule that recognizes an antigen is called an antibody.
An antigen-antibody reaction is exquisitely specific. This permits the unequivocal detection of a protein. As well, some antigen-based methods are highly sensitive, and so permit the quantification of very small amount of the protein antigen.
Antibodies are also referred to as immunoglobulins (Igs). Specific genes for antibodies direct the construction of antigen specific regions of the antibody molecule. Such antigen-specific regions are located at the ends of the arms of the Y-shaped immunglobulin molecule. The central core of the immunoglobulin is more constant in construction. Genetic engineering and the use of various mutational mechanisms allows the construction of a vast array of antibodies (each with a unique genetic sequence).
There are five different antibody types (Ig G, A, M, D, and E), each with a different Y-shaped configuration and function.
IgG is the most common type of antibody. It is the chief Ig against microbes. It acts by coating the microbe to hasten its removal by other immune system cells. It gives lifetime or long-standing immunity against infectious diseases. It is highly mobile, passing out of the blood stream and between cells, going from organs to the skin where it neutralizes surface bacteria and other invading microorganisms. This mobility allows the antibody to pass through the placenta of the mother to her fetus, thus conferring a temporary defense to the unborn child.
The antibody responsible for allergic reactions, IgE, acts by attaching to cells in the skin called mast cells and basophile cells (mast cells that circulate in the body). In the presence of environmental antigens like pollens, foods, chemicals, and drugs, IgE releases histamines from the mast cells. The histamines cause the nasal inflammation (swollen tissues, running nose, sneezing) and the other discomforts of hay fever or other types of allergic responses, such as hives, asthma, and in rare cases, anaphylactic shock (a life-threatening condition brought on by an allergy to a drug or insect bite). An explanation for the role of IgE in allergy is that it was an antibody that was useful to early man to prepare the immune system to fight parasites. This function is presently overextended in reacting to environmental antigens.
The presence of antibodies can be detected whenever antigens such as bacteria or red blood cells are found to agglutinate (clump together), or where they precipitate out of solution, or where there has been a stimulation of the plasma complement system. Antibodies are also used in laboratory tests, including the analysis of forensic samples, for blood typing and for the identification of target microorganisms or toxins.
The use of antibodies in forensic investigations is also called forensic serology. Blood typing is a common example of forensic serology. Here, antibodies against the A or B proteins that can be present on the surface of a blood cell are used to differentiate the four types of blood (A, B, AB, and O). If blood cells have only the A antigen present, then in the presence of the anti-A antibody, the cells can agglutinate. However, in the presence of anti-B antibody, which does not recognize the antigen, the cells will not agglutinate.
Antibodies are also used to discriminate blood from someone with a blood-related malady (i.e., sickle-cell anemia), based on the presence or one or more abnormal enzymes in the blood.
Other forensic serology applications include the detection of drugs, noxious compounds like toxins, and past exposure to specific microorganisms.
SEE ALSO Analytical instrumentation; Antigen; Biosensor technologies; Immune system.