Schematic representation of classical immunoassays that measure levels of nonantibody serum proteins and an assay that measures levels of antibody analytes. Schematic representation of (A) classical immunoassays that use antibody reagents to measure the levels of nonantibody serum proteins and (B) an assay that measures levels of antibody analytes. In (A), off-target binding of a nondisease-related protein will usually not generate a signal since the sandwich antibody is unlikely to bind to this protein. However, any nonspecific binding of an IgG ligand to nondisease-related antibodies will generate a signal, since the binding of any IgG is registered by the labeled secondary antibody. This places a high premium on selective ligands.