Bioimmunoassay (BIA): A Sandwich Immunoassay Scheme Employing Monoclonal Antibodies and Hormone Receptors to Quantify Analytes

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Abstract
When some antigens bind to receptors, a portion of the antigen remains exposed and can be recognized by labeled monoclonal antibodies. By measuring the amount of antibody bound to the antigen-receptor complex, one can quantify the amount of antigen that is present. Since this assay procedure depends on simultaneous receptor recognition of a biologically active site and antibody recognition of a distal epitope on the analyte, we call it a bioimmunoassay. Bioimmunoassays have many of the advantages of radioligand receptor assays (RRA) used to quantify biological activity and, depending on the choice of antibodies employed, may be more specific than RRA. In addition, since they are sandwich assays, they are usually more sensitive than RRA. Bioimmunoassays can be performed in several different modes and in the case described here we used a radiolabeled antibody to detect hormone-receptor complexes. Hence we term this example a bio-immunoradiometric assay or BIO-IRMA. We illustrate the properties of various assay procedures using a monoclonal antibody to the beta subunit of hCG which recognizes an epitope common to all other mammalian LH/hCG-like gonadotropins and which is capable of detecting 10 pg of hCG standard. In principle, this assay can be applied to any material capable of binding to a receptor, enzyme, etc. which can also be recognized by an antibody. Since it is a sandwich type of assay, it is subject to the same advantages and limitations of other sandwich assays except that it can be used to discriminate some biologically active and inactive analytes. Monoclonal antibodies which are prepared from spleen cells of animals immunized with antigen-receptor complexes and selected for their ability to bind antigen-receptor complexes should prove most useful for bioimmunoassay procedures.