Constant regions/segments

The p-constant region segment is a component of all newly rearranged immunoglobulin genes, but later in the life span of the cell, there is yet another recombination event called heavy-chain switching in which the p chain is deleted and a g chain is joined to the same V-D-J segment. The p- and (d-) chain antibodies are membrane-bound, whereas the g-chain antibodies (as well as other heavy-chain antibodies) are secreted by the cell. 

Stiemholm, N.B.J., Verkoczy, L.K., Berinstein, N.L. (1995). Rearrangement and expression of the human /CX6 gene segment results in a surface Ig receptor with a truncated light chain constant region. J. Immunol. 154,4583-4591. 

The second step in the formation of the heavy chain involves transcription of the DNA to form the primary transcript of the RNA of the developing lymphocyte, which includes the new VDJ sequence. This primary transcript (tRNA), however, still includes non-codifying segments (introns). In the third step, these introns are eliminated, and the final RNA includes only the variable N-terminal gene and the genes for all of the constant regions. 

Another alternative is the insertion of small sequences of human chromosomes into embryonic animal cells, thus generating trans-chromo-somic mice. These mini chromosomes are isolated from human chromosomes 2 and 14, which contain the genes for the light and heavy chains, respectively. This means that all of the V, D, and J segments of the variable N-terminal portion, as well as those of the constant regions, will become part of the mouse genome (van Dijk and van de Winkel, 2001 Roque et al., 2004). 

Figure 14.2 Immunoglobulin gene pools found in mammals (not drawn to scale) V, variable-region genes J, joining segment C, constant region hatched areas are introns and leaders.

Flanagan, J.G. and Rabbits, T.H. (1982). Arrangement of human immunoglobulin heavy chain constant region genes implies evolutionary duplication of a segment containing... 

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