Schematic representation of membrane bound IgM and secretory IgM. B lymphocytes display membrane bound IgM (mIgM)—shown left—on their surface as the core part of the BCR. Once committed to the plasma cell stage they secrete secretory IgM (sIgM) in either “pentameric”—shown right—or “hexameric”—not shown—form. HCs and LCs consist of various Ig-fold domains VH, CH1–4; VL, CL that are color-coded as indicated. Glycans and intra- and intersubunit disulfide bonds between HCs, LCs, and J-chain are depicted. Note that the Ig- HC (in mIgM) differs from the Ig- HC (in sIgM) in their C-termini, having a transmembrane domain (TMD) or, respectively, a cysteine containing tail piece (TP) as indicated.
