Hapten Contact and Hypervariable Regions

Capra et al. (1971) examined the light chains from some homogeneous antibodies derived from patients with hypergammaglobulinemic purpura. All these antibodies had IgG-ligating activity. The amino acid residue sequence was identical in some up to residue 40 between others 

These similarities are not surprising. They may represent, for instance, the retention of certain H- and L-chain sequences which ligate some persisting pathogens and thus be subject to selective pressure. Alternatively, they may represent an example of parallel evolution or the development of the same ligating sequence from two originally different sequences under the selective pressure by, perhaps, a common pathogen. 

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