{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"IgM forms polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, normally as a pentamer or occasionally as a hexamer. It has a large molecular mass of approximately 900 kDa (in its pentamer form). In germline cells, the gene segment encoding the constant region of the heavy chain is positioned first among other constant region gene segments. For this reason, IgM is the first immunoglobulin expressed by mature B-cells. IgM nephropathy is an uncommon glomerular disease characterized by IgM deposits in the mesangium."}]}]}