The lingering enigma of the allelic exclusion mechanism

B lymphocytes produce diverse antibody specificities by randomly assembling antibody genes from germline segments. Yet, though each B lymphocyte has multiple allelic loci for the different antibody chains, each clonally derived mature B lymphocyte expresses a single species of antibody with a unique specificity via a process termed allelic exclusion. Despite some progress, the precise mechanism of allelic exclusion remains an enigma.