Open sandwich immunoassay (OS-IA) is a relatively new immunoassay that employs the dependency of the interaction between the separated heavy-chain and light-chain variable regions of an antibody on the presence of an antigen. The interaction is enhanced when the two fragments bind to the antigen at the same time, resulting in their association, whereas they are prone to dissociate in the absence of the antigen. Since its first report in 1996, the assay principle has been further studied and successfully applied to the detection of various antigens, especially small molecules. The OS-IA can be conducted with the two fragments derived from one antibody, and can be carried out with less handling and shorter time than required for the sandwich immunoassay. It detects antigens noncompetitively, and with a higher sensitivity and broader detection range than those of competitive immunoassays. This principle has been applied to many small molecules, including pesticides, medicines, hormones, and oxidized lipids, allowing its application for monitoring pollutants in the environment and diagnosing diseases through biomarker detection. By employing the OS selective method, antibodies with higher affinity and OS assays with higher sensitivity can be developed. Moreover, in combination with phage display technology, effective and fast methods for screening antibodies suitable for OS-IA have been developed. Although some antibodies against large molecules (e.g., proteins) are not suitable for this immunoassay, in some cases, the interaction between the two variable regions can be changed to enable antigen detection with OS-IA, by altering the amino acid on the interchain surface of the variable region through protein engineering. Furthermore, the OS-IA principle has been applied to a micrometer-scaled device, giving many merits, such as the requirement of only a small sample volume, rapidity, and high sensitivity. The principle, recent achievements, and applications of OS-IA detection systems are introduced in detail in this chapter, and the strategies for faster and more sensitive configurations of this assay are reviewed.
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