Probing antibody-antigen interactions

Antibodies are biologicalmolecules generated by the host immune system in response to the invasion of foreign bodies or antigens. Therefore, antibodies must possess high specificity toward target antigens in order for the antigen to be recognized and subsequently destroyed. Because of this specificity, antibodies or antibody fragments that maintain binding specificity are heavily used in diagnostic assays and are becoming increasingly important in many therapeutic applications. Classical immunoassays such as radioimmunoassay and enzyme-linked immunosorbent assay are effective analytical techniques that have been widely used to screen and determine antibody specificity. Because of increased demands for antibodies with well-defined specificities, other techniques have been developed that facilitate generation and characterization of antibody-binding specificities under different conditions, such as when the protein is expressed on a cell surface or the target antigen is hard to isolate. Here, we describe three alternate techniques that provide unique abilities to characterize antibody-antigen binding events: (i) surface plasmon resonance, (ii) fluorescence activated cell sorting, and (iii) atomic force microscopy. These different techniques take advantage of various changes in physical and/or chemical properties of the analytes that occur upon binding, such as refractive index, surface charge, and changes in structure. These techniques provide unique powerful advantages over traditional immunoassays including real-time and label-free detection, low sample volume and concentration requirements, and molecular-level detection sensitivity. This article provides an overviewof howthese alternate approaches to studying antibody-antigen interactions can be used to facilitate rapid development of new antibody-based reagents for diagnostic and therapeutic applications.