Antibody and antigen are defined in terms of each other. An antibody is an immunoglobulin, with a specific amino acid sequence, which is produced by plasma cells (terminally differentiated B-cells) in response to specific recognition of a foreign, nonself molecular configuration, the antigen (immunogen). Each antibody has a specific affinity for the antigen that stimulated its synthesis. The body has a nearly unlimited ability to produce different antibodies against specific antigens. However, the specific affinity of an antibody is not for the entire macromolecular antigen, but only for a particular site on the antigenic molecule, called the epitope or antigenic determinant, that selects and defines the binding site of the antibody to which it is complementary (Section 220.127.116.11). Thus, the specificity in the antigen-antibody reaction is of a uniquely structured antibody against a unique epitope. Some cross-reactivity can occur if a particular molecule mimics an epitope too closely.
Monoclonal antibodies (MoAbs) are specific antibodies that are chemically and immunologi-cally homogeneous. They derive from a single clone of cells in certain neoplasms, of plasma cell or B-cell origin, or by using hybridoma techniques that involve fusing, in cell cultures, specific antibody-producing lymphocytes of limited life span with immortal tumor plasma cells. Because of their high specificity in binding to target antigens, MoAbs from hybridomas are now used in diagnosis, as immunological probes in a variety of assays, and in therapy, to deliver radiation (yoked to radionucleotides) or other therapy directly to cancerous tissues.
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