Allelic exclusion

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To begin with, it is important to remember that humans are diploid, i.e. they have two copies of each chromosome (one inherited from each parent) in all their somatic cells. Both chromosomes within the pair contain essentially the same genes, but these genes may contain a small number of nucleotide differences. The same gene on a paired chromosome is called an allele. Allelic exlusion is a process where a protein product from a recombined gene (e.g. a T cell receptor-beta chain or immunoglobulin/antibody Heavy or Light chain genes) from one chromosome actively prevents any further rearrangement and recombination of the same gene locus or allele on the second chromosome within a pair. In other words, no other combinations of gene segments for the antigen receptors will occur within an individual cell after a successful recombination event has occurred within that cell. Allelic exclusion, therefore, ensures that all the antigen receptors (immunoglobulin on B cells, TCR on T cells) on an individual lymphocyte will have the same amino acid sequence in their variable domains and thus will have identical antigen specificities. This process occurs within the individual cells and is the reason why there is such diversity in the antigen receptor repertoire within the entire population of B and T lymphocytes.