Immunoassays – Immunometric	

	For the measurement of larger antigens with several different antibody binding sites, the immunometric or sandwich assay technique is more often used. In the former technique, the label is coupled to one of the antibodies (Fig. 22.13). This assay is a reagent excess assay and requires shorter incubation times than competitive assays. Two different monoclonal antibodies are used for this type of assay: one is fixed to a solid phase, usually the wall of the reaction vessel, while the other is coupled to the label. During the immunological reaction, the antigen binds the solid-phase antibody on one epitope and the labelled antibody on another. The so-formed ‘sandwich’ complex is then separated from non reacted components.