Structural basis for Fc receptor recognition of immunoglobulin M

FcR is the IgM-specific Fc receptor involved in the survival and activation of B cells. Using cryo-EM, we reveal eight binding sites for the human FcR Ig domain on the IgM pentamer, one of which overlaps with the receptor binding site for the transcytosis receptor pIgR, but a different mode of binding explains Ig isotype specificity. The complex explains engagement with polymeric serum IgM and the monomeric IgM B cell receptor.