What is allelic exclusion and why is it important?

Allelic exclusion is defined as a process in which only one allelic gene is expressed and other allele remains silenced.

Allelic exclusion plays important role for cell surface receptors in genes and  in immune cells like B lymphocytes. The allelic exclusion controls binding site of B cells and allows only one type of binding site expressed in each B cell as    binding site of an antibody is decided by the combination of a particular heavy and light chain.