The present invention relates to the field of diagnostic or analytic methods. In particular, the inventors teach that an Fc receptor for IgM heavy chains (FcμR), in particular, Faim 3/Toso or CD351, or a functional fragment thereof, which preferably comprises the Ig-like domain of Faim 3/Toso, may be used for in vitro quantification of IgM or IgA antibodies, preferably, IgM, in the form of immune complexes, i.e., complexes formed by antigen and specific antibody. These immune complexes may be formed by mixing an antigen with a sample comprising antibodies. A method for detection and quantification of IgA or IgM antibodies in the form of immune complexes is also provided. This can be useful for diagnosis of infections, e.g., with a virus.