The old but new IgM Fc receptor (FcμR).

IgM is the first Ig isotype to appear during phylogeny, ontogeny and the immune response. The importance of both pre-immune "natural" and antigen-induced "immune" IgM antibodies in immune responses to pathogens and self-antigens has been established by studies of mutant mice deficient in IgM secretion. Effector proteins interacting with the Fc portion of IgM, such as complement and complement receptors, have thus far been proposed, but fail to fully account for the IgM-mediated immune protection and regulation of immune responses. Particularly, the role of the Fc receptor for IgM (Fc mu R) in such effector functions has not been explored until recently. We have identified an authentic Fc mu R in humans using a functional cloning strategy and subsequently in mice by RT-PCR and describe here its salient features and the immunological consequences of Fc mu R deficiency in mice. Since the Fc mu R we cloned was identical to Toso or Fas inhibitory molecule 3 (FAIM3), there have been spirited debates regarding the real function of Fc mu R/Toso/FAIM3 and we will also comment on this topic.