Antibodies as evidence
Antibodies are specialized proteins produced by the immune system, primarily by B lymphocytes, and play a crucial role in identifying and neutralizing foreign substances such as pathogens and toxins. In forensic science, antibodies are invaluable tools used to detect and analyze biological evidence, including human blood, semen, and saliva, as well as various drugs of abuse. Their ability to bind specifically to antigens—substances that trigger immune responses—enables forensic scientists to conduct tests that are often quicker and more cost-effective than alternative methods.
There are two main types of antibodies used in forensic applications: polyclonal antibodies, which are derived from the blood of immunized animals and can recognize multiple sites on an antigen, and monoclonal antibodies, which are produced from a single clone of lymphocytes and provide highly specific recognition of particular antigens. These antibody-based tests are not only essential in criminal investigations but are also employed in employment drug screening and sports anti-doping measures. Additionally, antibodies facilitate improvements in DNA testing techniques, enhancing the accuracy of analysis in forensic investigations. Overall, the use of antibodies significantly advances the capabilities of forensic science in identifying and interpreting biological evidence.
Antibodies as evidence
DEFINITION: Group of interacting cells and molecules inside a host organism whose function is to recognize and eliminate or neutralize foreign material such as pathogens or substances that enter the host organism.
SIGNIFICANCE: Antibodies, which are products of the immune system, are used in many tests to detect, identify, and measure substances of interest—such as drugs, different types of biological evidence, and certain toxins—during criminal investigations. Without such antibody-based tests, some types of testing performed by forensic scientists would not be possible, and others would be much more expensive and time-consuming than they are.
The organization of the immune system can be described in various ways based on the physical and functional characteristics of the system’s components. For example, the immune system can be divided into humoral (soluble molecules such as antibodies and complement) and cellular immune systems (white blood cells, or leukocytes). Another way to describe the immune system is to distinguish between the adaptive (or acquired) immune system and the innate (or natural) immune system.
The adaptive immune system exhibits a lag phase before response and great specificity for particular foreign materials; it has a “memory” for foreign materials and responds more quickly and strongly upon subsequent encounters. Prominent components of the adaptive immune system are white blood cells called T and B lymphocytes and molecules such as antibodies and T-cell antigen receptors. The natural immune system’s response is immediate, does not alter in magnitude when reencountering foreign material, and operates against broad classes of materials. Prominent components of the innate immune system include molecules of the complement system and cells such as monocyte/macrophages, neutrophils, natural killer cells, eosinophils, mast cells, and basophils.
Cells and Functions
The cells of the immune system include different types of white blood cells that are also known as leukocytes. They all originate from the bone marrow and are found in tissues throughout the body as well as in specialized immune tissues such as lymph nodes and vessels, spleen, appendix, and tonsils. The cells have shared and unique functions such as engulfment of foreign particles (phagocytosis), release of microbe-killing molecules, and secretion of molecules called cytokines that affect leukocytes and other cells.
Cells such as B and T lymphocytes are capable of undergoing clonal expansion. That is, these lymphocytes consist of populations of distinct cells, each bearing a particular surface receptor capable of binding a particular foreign substance—an antigen. Upon binding a particular foreign substance, such a lymphocyte can divide to produce copies of itself (clones) that can produce more of the corresponding receptor for the foreign substance. In order to manipulate the immune system in the test tube or the body to produce substances useful in forensic testing, scientists employ detailed knowledge of the properties of immune system cells.
Antigens and Antibodies
Antigens are substances that provoke immune system responses, such as the generation of cells or antibodies that can specifically bind the antigens. Forensic scientists frequently encounter a number of antigens in their work, including hemoglobin, prostate-specific antigen (PSA), alpha amylase, and semenogelin. Other forensically significant antigens are carbohydrate substances such as ABO blood group antigens and Lewis antigens. Also of interest to forensic scientists are small parts of antigens known as haptens, which include drugs of abuse such as cannabis, heroin, cocaine, and and their metabolites.
Antibodies are a class of proteins that are synthesized and secreted only by B lymphocytes, a subpopulation of white blood cells. The five classes of antibodies (termed IgG, IgM, IgA, IgE, and IgD) all have different structural and functional characteristics.
Antibodies are Y-shaped proteins, and the basic unit is composed of four polypeptide chains, two heavy chains and two light chains. Both light and heavy chains have “variable” regions that differ in composition among antibodies and “constant” regions that are identical or similar within a particular class of antibodies. The site on antibodies that binds antigen encompasses the tips of one heavy chain and one light chain in the arms of the Y-shaped antibody and involves the variable regions. The constant regions, depending on the class, may activate the complement system to lead to removal or destruction of materials foreign to the body or cause leukocytes to engulf antigens or secrete biologically active molecules.
To create antibodies for use as tools in forensic testing, a particular substance may be injected into an animal to elicit specific antibodies to that substance. (Among the animals commonly used for such purposes are mice, rats, rabbits, goats, and sheep.) The antibodies, termed polyclonals, can be isolated from the animal’s blood and used as components in forensic tests for that substance. Such antibodies vary in potency.
Monoclonal antibodies are produced through the isolation of a particular lymphocyte secreting a particular antibody and propagation of the clones of that lymphocyte in culture. This provides unlimited amounts of a reproducible specific antibody to a particular antigen.
Forensic Applications of Immunology
In forensic science, antibodies are used extensively in the areas of serology and toxicology. In serology, antibodies are used in tests to detect and identify human blood, semen, and saliva stains. In toxicology, antibodies are used in tests for poisons and for drugs of abuse. Like the drug tests performed by forensic scientists during criminal investigations, the tests widely used by some employers and sports leagues to screen employees and players for drugs of abuse and banned performance-enhancing substances are also antibody-based.
Another example of the use of antibodies in is found in the area of (deoxyribonucleic acid) analysis. Forensic scientists use a technique known as Polymerase chain reaction (PCR) to make it possible to test minute quantities of DNA in evidence. In one version of the technique called hot-start PCR, the accuracy of the process is increased by the inclusion of an antibody to the DNA polymerase used.
Research shows that
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