IgM hexamers?

There has been a universal tendency to regard IgM antibodies as pentameric molecules comprising five immunoglobulin monomeric subunits joined by a single J-chain protein. Is this the only form of secreted IgM, or are the possibilities more complex? In this article, Joseph Brewer and colleagues propose that the IgM polymers secreted in primary immune responses may be more heterogeneous than previously believed and that, as a consequence, the biological activity of IgM may have considerable flexibility.