Figure 3: Receptors that bind sIgM.

The polymeric Ig receptor can bind pentameric IgM as well as dimeric IgA via binding to the J-chain, resulting in transcytosis through epithelial cells located on mucosal surfaces and luminal excretion. Fc$\alpha /\mu$R: This receptor is expressed predominantly by macrophages and follicular dendritic cells, where it binds to IgA and IgM with moderate and high affinity, respectively, to mediate endocytosis of IgA/IgM-antigen complexes. Complement Receptors: Complement receptor 1 is the C3b/C4b receptor is expressed on a variety of immune cells, including B cells and follicular dendritic cells (FDC), where it anchors sIgM-antigen-C3 complexes onto FDCs. Complement receptor 2 binds to iC3b (inactive derivative of C3b), C3dg and/or C3b. This receptor is expressed on B cells as well as FDCs. Fc$\mu$R: The Fc$\mu$R is highly expressed on B cells and binds sIgM selectively, which is rapidly internalized. The functional consequences of FcμR/sIgM interaction remain to be fully resolved.