Final answer:
In immunology, allelic exclusion is a phenomenon that guarantees a B or T cell expresses a unique antigen receptor. The process occurs in T-cell receptor α genes, and B-cell receptor heavy-chain, κ-chain, and λ-chain genes, but may not strictly apply to T-cell receptor β genes because multiple recombination events can occur until a functional gene configuration is achieved.
Step-by-step explanation:
Allelic exclusion is a process whereby only one allele of a gene is expressed while the other allele is silenced. This ensures that each B or T lymphocyte expresses a unique receptor. In the context of the immune system, this contributes to the diversity of the antigen receptors. All the options listed (a through e) are sites of allelic exclusion except for 'b. T-cell receptor β genes.' T-cell receptor (TCR) α and β chains both undergo allelic exclusion. However, TCR β gene rearrangement happens first. If successful, allelic exclusion ensures that only one β chain is produced. If the rearrangement is unsuccessful, the other TCR β allele is recombined to attempt a productive rearrangement. Consequently, not all TCR β genes are subject to allelic exclusion as multiple attempts can be made to achieve a functional gene rearrangement. In contrast, allelic exclusion applies rigorously to B-cell receptor heavy-chain, κ-chain, and λ-chain genes ensuring that each B-cell expresses a single specificity.