The parts of the heavy or light chains of immunoglobulin molecules that are of constant sequence, in contrast to variable or V regions. The constancy of sequence is relative because there are several constant region genes and alleles thereof (see allotypes), but within one animal homozygous at the light and heavy chain constant region genes all immunoglobulin molecules of any one class have constant sequences in their C regions. The constant region sequences for the various different types of immunoglobulin eg. IgG, IgA, etc. will vary.
Dictionary of molecular biology. 2004.