My drawing of the IgM-BCR contains two heavy chains, two light chains, and an Igα/Igβ heterodimer. The drawing also contains 14 glycosylation sites, additional labels of Fab, the constant and variable regions, and the Ig-like extracellular domains (ECD). The IgM-BCR is crucial for the development and activation of B cells. Its ability to detect antigens with help from the plasma membrane also plays a role in recognizing antigens with the cluster of BCRs interacting with the extracellular domain and the intracellular membrane. The molecule differs from the soluble pentameric IgM due to their structural variations. The soluble pentameric IgM contains two C-terminal β strands of the Cμ4 that mediate oligomerization. In contrast, the Cμ4 of the IgM-BCR has C-terminal α strands and transmembrane helices (TMμ_A and TMμ_B) anchor the receptor onto the cell membrane.