Fcα/μ receptor mediates endocytosis of IgM-coated microbes

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Abstract
IgM is the first antibody to be produced in a humoral immune response and plays an important role in the primary stages of immunity. Here we describe a mouse Fc receptor, designated Fcα/μR, and its human homolog, that bind both IgM and IgA with intermediate or high affinity. Fcα/μR is constitutively expressed on the majority of B lymphocytes and macrophages. Cross-linking Fcα/μR expressed on a pro-B cell line Ba/F3 transfectant with soluble IgM or IgM-coated microparticles induced internalization of the receptor. Fcα/μR also mediated primary B lymphocyte endocytosis of IgM-coated Staphylococcus aureus. Thus, Fcα/μR is involved in the primary stages of the immune response to microbes.