Allelic Exclusion -
When a B cell differentiates and matures into a plasma cell, it synthesizes solitary types of light and heavy chains. This happens despite the presence of two loci (one paternal and one maternal) for heavy chain and four loci (two paternal and two maternal) for light chain in a diploid cell.

This phenomenon, where some of the loci are excluded from being expressed is known as alletic exclusion and is unique to antibody gene expression. In the expression of antibody genes, VH segments are rearranged first, and then the genes for k light chain segments are assembled.

The λ light chain genes are assembled only if rearrangements at the V к locus fail to produce a functional k light chain. The rearrangements of VH and V k genes thus inhibit any further rearrangement.