From absolute to exquisite specificity. Reflections on the fuzzy nature of species, specificity and antigenic sites.

The term specificity is derived from the word species and shares with it an inherent fuzziness based on the absence of sharp boundaries between closely related entities. Antibody specificity is a ternary relational property which refers to the antibody's capacity to discriminate between two or more epitopes. There are no sharp boundaries between the individual overlapping epitopes that constitute an antigenic site and there is also no clear-cut minimum difference in binding affinity or in atomic positions at the epitope-paratope interface that can serve as a yardstick for deciding that two epitopes or two paratopes are the same or not. Immunology shares with the whole of empirical science the need to handle fuzzy sets and concepts and this poses no threat to the unabated further development of immunochemical analysis.