Many traditional tests used in forensic biology are immunological in nature. A brief account of basic immunological principles is given before discussing the tests.

Antibodies are proteins, called immunoglobulins (Ig), which are produced by white blood cells (WBC) in response to stimulation by foreign materials (antigens). They are found in serum in the gamma globulin fraction. All immunoglobulins have the same basic structure consisting of two pairs of peptide chains linked to form a Y-shaped molecule. The chains in the longer pair are designated as "heavy" or H and those in the shorter pair as "light" or L chains. The chains within each pair are identical. However, these pairs (H and L) differ from each other. There are five classes of immunoglobulins, each of which are differentiated by their chemical structure.

The antibodies involved in precipitin reactions are mainly IgG, whereas IgM molecules are the class responsible for agglutination reactions.

Antigens (also referred to as immunogens) are variously described as compounds that stimulate production of antibodies in an immune response or as substances that combine with an antibody. Immunogens are usually large molecules, such as proteins. A chemical complex is formed when an antibody binds to the epitope region on an antigen. It is possible to have an antibody-antigen reaction where the antigen would not produce an immune response except in combination with a carrier molecule; such an antigen is correctly termed a hapten.

The immune response to an antigen challenge is the production of antibodies. Each cell line produces identical antibodies; cultures of these cells will generate highly pure and specific monoclonal antibodies. Monoclonal antibodies interact with a specific epitope region, whereas polyclonal antibodies interact with numerous sites on the antigen. Each epitope on the antigen produces specific antibodies. Serum containing antibodies is called antiserum.

The antibody-antigen reaction is specific. This can be visualized as a lock and key; the binding site on the antibody fits exactly with the epitope region on the antigen. For example, each IgG is a bivalent antibody having two identical receptor sites specific to an epitope on the antigen. This enables cross-linking to occur.

This is the basis of the precipitin and agglutination reactions.  In the former, divalent IgG molecules cross link with binding sites on proteins to form a high-molecular weight, insoluble precipitate.  In the latter, the IgM molecules exist as pentamers that are able to cross link with binding sites on the cell surface, resulting in clumping, or agglutination, of the cells.  In both cases (precipitin and agglutination), the aggregates can be seen with a low power microscope or the naked eye.

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