Variable region sequences of five human immunoglobulin heavy chains of the VH3 subgroup: definitive identification of four heavy chain hypervariable regions.

The variable regions of five human immunoglobulin heavy chains of the V(H)III subgroup have been totally sequenced. Three of the heavy chains belonged to the IgG class and two to the IgA class. Examination of these sequences, and comparison with additional published heavy chain sequences, showed that a total of four hypervariable regions is characteristic of human heavy chain variable regions. The relatively conserved character of large segments of the heavy chain variable region was very evident in these studies. The conserved segments, which are those sections located outside the hypervariable regions, comprise approximately 65% of the total heavy chain variable region. The following general structural pattern for antibody molecules emerges from this and related studies: an overall combining region superstructure is provided by the more conserved segments while the refinements of the active site specificity are a function of hypervariable regions.