IgM forms polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, usually as a pentamer or a hexamer. It has a large molecular mass of approximately 900 kD. The J chain is attached to most pentamers, while hexamers do not possess the J chain due to space constraints in the complex. Because each monomer has two antigen binding sites, an IgM has 10 of them, however it cannot bind 10 antigens at the same time because they hinder each other. Because of its large molecule, it cannot diffuse well, so it is found in the interstitium only in very low quantities. IgM is primarly found in serum, however of the J chain it is also important as a secretory immunoglobulin. Due to its polymeric nature, IgM possesses high avidity, and is particularly effective at complement activation. It is also a so-called "natural antibody". This means that it is found in the serum without an evident contact with antibody.