Serves to link two monomer units of either IgM or IgA. In the case of IgM, the J chain-joined dimer is a nucleating unit for the IgM pentamer, and in the case of IgA it induces dimers and/or larger polymers. It also helps to bind these immunoglobulins to secretory component. Part of the secretory IgA (sIgA) complex that consists of two, four or five IgA monomers, and two additional non-Ig polypeptides, namely the JCHAIN and the secretory component (the proteoytic product of PIGR).