Antigenic sites definition

The exact boundary, residue, conformational, and directional definitions of the three antigenic sites of hen egg-white lysozyme have been described. The results revealed that the three antigenic sites account quantitatively for the total antigenic reactivity of the protein. Thus the entire antigenic structure of the enzyme has now been determined precisely. Its nature was discussed together with the power of the surface-simulation synthetic concept. 

Definition of the association (or avidity) constant for such multivalent antibody—antigen reactions must consider not only the heterogeneity of the antibodies and the antigen determinant site(s), but also an apparent additive effect of binding two antigen molecules to a single antibody. Such effects lead... 

Problems may also arise with respect to specificity when ELISAs are applied for trace residue analysis, because any compound whose molecule is in part identical with or closely similar to the antigenic determinant of the analyte can compete for antibody-binding sites. Therefore, immunochemical methods are valuable for screening and testing purposes but cannot be considered as definitive from a regulatory perspective. For legal enforcement use, these methods should be used as part of an analytical system that consists of additional methods capable of definitively identifying the compounds of interest. 

Now, while it is possible, by lowering the surface tension of the liquid medium to 50 dyn/cm by means of solutes of low molecular weight, to dissociate antigen-antibody precipitates (of the pure VDW type) (6), whole mammalian serum (with a surface tension < 50 dyn/cm) definitely has no such dissociating power. Upon some reflection this is not as anomalous as it might seem. Table I shows that it is mainly because of the presence of proteins (of molecular weight > 20,000) that blood (or plasma) has a surface tension y < 50 (and not y 70), and it cannot be expected that molecules with dimensions of the order of 100 A will effect a separation by purely physical means between other proteins of the same approximate size that are only about 2 A apart at their site of interaction (13). 

All enzymes are in fact labeled, at least by their active binding ate for substrates, and by coenzymes (catalytic sites) which both together enable them to fulfil their very specific catalytic function. The same can be said about hormones and antibodies which are adapted for a specific interaction with a target cell or antigen by some definite mode of labeling. 

Low selectivity is another problem that is encountered in immunoaffinity chromatography. Obviously, if an antibody shows little preference for one antigen over another, no separation will be achieved. By definition, a monoclonal antibody recognizes — and binds to -- a single site and is therefore highly selective. As implied by their name, switch monoclonal antibodies are selective and, under the right circumstances, are... 

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