Allelic exclusion of lymphocyte antigen receptor chains has been hypothesized as a mechanism developed by the immune system to ensure efficient lymphocyte repertoire selection and tight control of lymphocyte specificity. It was effectively shown to be operative for both the immunoglobulin (Ig) and the T cell receptor (TCR) beta chain genes. Our present observations suggest that close to 1% of human T lymphocytes escape this allelic control, and express two surface TCR beta chains with distinct superantigenic reactivities. Since this high frequency of dual beta chain expressors did not result in any dramatic immune dysregulations, these results question the need for a mechanism ensuring clonal monospecificity through allelic exclusion.

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