The last 30 years of the biotechnology revolution have to a great extent been fueled by the discovery and application of monoclonal antibodies for research and therapeutic purposes . Antibodies are Y-shaped molecules produced by the immune system of many vertebrates to recognize and neutralize foreign pathogens or toxins with very high specificity. The highly specific nature of antibody-antigen interactions provides a means of molecular targeting that is extremely useful both as scientific reagents and as clinical therapeutics. An antibody molecule is comprised of two major portions: the constant region (or Fc region), which does not vary among distinct antibody molecules with different specificities, and the variable region, which comprises a unique sequence for each antibody and is the region responsible for antigen recognition (Fig. 1.1). Binding to molecular targets occurs at the exposed ends of the variable region. Different antibody variable regions confer distinct antibody binding specificities conferred by their unique antibody amino acid sequences, which are in turn derived from somatic recombination of the immunoglobulin genes and subsequent clonal selection that occurs during B-cell development.