Antibody - antigen interactions complex formation

The attachment of protein to the transducer surface for immunoassay is a difficult problem, since it must be achieved without interfering with the active site. If an unlabeled assay is to be performed it must also be such that subsequent non specific interaction with the surface can be inhibited. This requirement is often contrary to those for effective antibody-antigen complex formation. A surface close packed with antibody will be sterically hindered and its reaction with antigen inhibited. On the other hand, a suitably spaced packing allows non-specific interactions to occur and large false positive signals to be recorded. As mentioned earlier, this was demonstrated by Cullen and Lowe who used the surface plasmon resonance technique to probe specific and non-specific protein interactions at metal surfaces. (23). 

Precipitin tests As the name implies, precipitin tests rely on the fact that when the appropriate ratio of antibody and antigen are mixed together, immune complexes of antibodies and soluble antigens come out of solution, settling to form a visible precipitate. The antibody-antigen precipitate formation can be plotted as a curve, and interaction is maximal at the top of the curve (termed the zone of equivalence) shown in Figure 10.2. This technique can be used to quantify the antibody content of a solution. 

AM methods, in contrast, depend on the modulation of the enzyme signal by competition of the test molecule for the same immunoreactant. Contrary to AA methods, the sensitivity of AM assays increases with lower immunoreactant concentrations since at low concentrations a variation in the amount of competing molecules has a larger impact on the interaction with the labeled species. According to the law of mass action, the low antigen and antibody concentrations reduce considerably the rate of complex formation. Moreover, at very low concentrations the accuracy tends to be poor. 

Bio- and electrochemists have come to give increasing attention to the interface between a charged surface and aqueous electrolyte solution in recent years. Many bioactive substances are known to be localized at particular points on biosurfaces where they perform various functions in vivo. The complex formation of an enzyme and its substrate, the specific binding of an antigen and its antibody, and the attachment of a neurotransmitter at a site on a biomembrane are typical cases of specific interaction on biosurfaces. 

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