The IgM drawing (although horrible) is a representation of the IgM BCR (IgM B-cell receptor). It shows all the important regions from the antigen-recognition portion, which is in the variable regions, to the glycosylation sites, which helps monitor the progress of protein folding. The IgM BCR’s function is to be an antigen-binding receptor that binds to specific antigens. This allows the B-cell to produce antibodies to help fight off the antigen. Although similar to soluble pentameric IgM, there is a key difference. The difference is that the soluble pentameric IgM lacks the transmembrane domain. The transmembrane domain allows the IgM BCR to hook on the B-cell allowing it to do its job more efficiently.