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The Fc receptor for IgA and IgM (Fcα/μR) is of particular interest because it can bind antibodies of both IgM and IgA isotypes and thus may play a pivotal role in systemic and mucosal immunity. Using IgM and IgA ligands and newly generated Fcα/μR specific monoclonal antibodies we have defined biochemical features and cellular distribution of the human Fcα/μR. Both recombinant and native forms of human Fcα/μR are expressed on the cell surface as remarkably stable homodimeric transmembrane glycoproteins that can bind specifically polymeric IgM or IgA. The only human B cells to express Fcα/μR, albeit at very low levels, are found in the pre‐germinal center subpopulation defined by the IgD+/CD38+ phenotype. Hence the expression pattern differs from that of the mouse wherein Fcα/μR is expressed by both circulating and resident B cell populations. Significantly, the predominant cell type expressing the Fcα/μR in humans is the follicular dendritic cell of germinal centers. The Fcα/μR may thus function in antigen presentation and B cell selection in the germinal center response.

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