Interference in Immunoassay

FIGURE I. Stylized diagram of the immunoglobulin molecule showing derivation of the various fragments used for preparation of antibodies in immunoassay. Immunoassays are a form of macromolecular binding reaction; no covalent chemical bonding is involved. Antibodies interact with their antigens by weak hydrogen bonding and van der Waals forces. Antigen-antibody reactions are dependent on complementary matching shapes (i.e. structural conformity) being assumed by the antibody variable (Fab) regions (see Fig. I) of the immunoglobulin. Almost all polyclonal antibodies used in immunoassay reactions are of the immunoglobulin G (IgG) class. The N-terminal 110 amino acid residues of both the heavy and light chains of the immunoglobulin molecules are variable in sequence and interact to form the antigen binding site. About 30 amino acids are potentially available as antigen contact residues, but X-ray crystallographic studies have shown that a maximum of only 17 amino acids are directly involved in binding contact. Nevertheless, this variability gives rise to a vast array of different