Abstract: The present invention relates to (1) a method for the detection and/or determination of an antigen specific immunoglobulin of a peculiar IgX class, in which X means M, A, D or E, comprising: (a) contacting and reacting the test-medium with either insolubilized anti-IgX against the antigen specific immunoglobulin of a peculiar IgX class to be determined, or antigen binding fragment of this anti-IgX, (2) incubating said contacted and reacted test medium with an antigen for which the immunoglobulin to be determined has specific affinity, and with a labelled antigen binding fragment of an antibody against said antigen, as well as (2) the novel coupling product comprising an antigen immunochemically bound to a labelled antigen binding fragment of an antibody against said antigen, and (3) test kit for the performance of said method (1). The labelling agent is detected and/or determined, which provides qualitative and/or quantitative information about the antigen specific immunoglobulin to be determined.