B-Cell Development: How to Become One of the Chosen Ones

One B lymphocyte makes one antibody, which it displays on the cell surface in 104–105 copies as antigen-recognizing B-cell receptors (BCRs). This central dogma of immunology is observed in 98%–99% of all B cells for the immunoglobulin (Ig) heavy (IgH) chain locus, and in 95%–97% of all Ig light (IgL) chain loci. Hence, in the majority of all B cells only one of the two IgHs, and only one of the four (in the mouse) or six (in humans) IgL loci are expressed. This choice of only one IgH and one IgL allele for the expression of protein is called “allelic exclusion.”