Waldenström macroglobulinemia (WM) is a monoclonal gammopathy, while it is not a plasma cell dyscrasia. Its clone is made of lymphoid cells that are not mature enough to initiate isotype switching, the change from the IgM heavy chain they first express to other immunoglobulin classes. Because the cells are not completely mature, the main protein that they make is IgM.
Because they are not mature enough, these cells can settle not only in the bone marrow but also in the lymph nodes, where they make masses. This is the reason that the other name for Waldenström macroglobulinemia is lymphoplasmacytoid lymphoma.
IgM differs from all the other forms of immunoglobulin in the pentameric form that it has, and the clinical consequences of WM depend on this form. The IgM pentamer can make the blood more hyperviscous in comparison to the same amount of IgG, for instance: it is much larger than an IgG molecule and stays inside the plasma rather than passing into the tissues, so even a moderate amount of it raises the viscosity of the blood and slows flow through small vessels.