Common origin and evolution of variable and constant regions of immunoglobulins.

Sequence data show that the immunoglobulins evolved from two sets of paralogous genes: a gene set coding for the V regions and another for the different C regions. A comparison of sequences from these two gene sets shows homology between the V and C sets of genes: this homology is only significant when VH is compared with Cmu1, Cmu2 and Cgamma1. There is a close agreement between our data drawn from sequence comparisons and the data of Poljak et al. (1974) drawn from crystallographic data. This finding is in agreement with the results of the phylogenetic trees of the C and V gene sets: they suggest that the VH subgroups and the first constant domain of the heavy chains are the most ancient. Moreover homology between the red blood cell glycophorin and Cmu2 suggests that immunoglobulins could have a common origin with some membrane proteins.