Expression of productively rearranged loci

The most immature B cells express membrane IgM. However, such cells often contain mRNAs for both the membrane and secreted form of the p polypeptide chain (termed pm and /us, respectively). The ratio of pm to ps mRNA varies considerably amongst B cell lines. Sometimes the two are in equimolar amounts, though often pm mRNA predominates. Nevertheless, whilst many B cell lines contain ps mRNA, secreted IgM is often not readily detectable this indicates the importance of translational or posttranslational regulation in determining the cell phenotype. 

More mature B cells often co-express surface IgM and IgD and there are rarer cells that show other combinations of heavy chain class expression. The later stages of B cell maturation and particularly the development into an IgM-secreting plasma 

---