Antibody-antigen complex formation

The binding strength between a single epitope and paratope is referred to as their affinity. It is dependent on the number and strength of the bonds formed between 

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Figure 1. Schematic representations of significant biological functions displayed by host-guest complexation in homogeneous solutions or at membrane surfaces, (a) Separation (e.g., antibody-antigen complex formation), (b) Transformation (e.g., enzymatic reaction), (c) Translocation (e.g., carrier- or channel-mediated transport), (d) Transduction (e.g., receptor-mediated transmembrane signaling).

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). 

Fluorescence Immunoassay. Basic FIA follows the same formats and approaches as EIA. The difference Hes in the indicator a fluotophote is used instead of an enzyme. This allows direct quantification of the indicatot—antibody—antigen complex, or free indicator-reagent, without the need for a substrate. 

Most immunoassay kits and many commercial immunoassay analyzers are based on heterogenous EIA or FIA. These include an immunoassay system that uses FIA linked to radial partition chromatography of the antibody—antigen complex (39) a system that uses antibody-coated tubes for enzyme immunoassay of a variety of hormones and dmgs (40) and a system that uses either a sandwich or competitive FIA format to measure a variety of analytes (41). 

Weak bonds, generally non-covalent ones, are involved in formation of the complex, so that the reaction is readily reversed. The rate of the forward reaction is given by the concentration of substrate multiplied by the rate of constant ku and rate of the reverse reaction is given by the concentration of the product multiplied by the rate constant 2- The dissociation constant for the ES complex is 2/ 1- This is analogous to the formation of other complexes for example receptor-hormone complex receptor-neurotransmitter complex antibody-antigen complex. 

First antibody-toxin complex formed I antibody-antigen complex formed Formation of viral peptides and MHC-peptide complex 1... 

Chemical - complex formation. Earlier we discussed the complexation which can occur between the chelating agent EDTA and heavy metals or the sequestration of lipids in cholestyramine resins. The immune system has antibody-antigen complexes formed which can help in the elimination of the antigen (xenobiotic). 

An important application of fluorescence is in immunoassays, which employ antibodies to detect analyte. An antibody is a protein produced by the immune system of an animal in response to a foreign molecule called an antigen. The antibody recognizes the antigen that stimulated synthesis of the antibody. The formation constant for the antibody-antigen complex is very large, whereas the binding of the antibody to other molecules is weak. 

The binding equilibrium expressed as shown above (2.2) is actually a gross oversimplification of the situation. The heterogeneity of the binding sites and multiple valency of individual antibodies lead to formation of secondary bonds that contribute to hysteresis or ripening of the antibody-antigen complex. Its ultimate form is the polymerization of a primary complex, which happens when the antigen is also polyvalent. Formation of the polymer (precipitin reaction) renders such a reaction virtually irreversible. 

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. 

The binding of phosphorylcholine does not significantly change the structure of the antibody, yet induced fit plays a role in the formation of many antibody-antigen complexes. A malleable binding site can accommodate many more kinds of ligands than can a rigid one. Thus, induced fit increases the repertoire of antibody specificities. 

Antibody/antigen complex itself may not adsorb and activate factor XII (31), but its formation leads to activation of complement factor Ce which in turn may activate clotting (40, 41). Here too, some products appear poorly soluble and may offer physical interfaces rather than chemical activity. 

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