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 kD (in its pentamer form). 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 it is a large molecule, it cannot diffuse well, and is found in the interstitium only in very low quantities. IgM is primarily found in serum; however, because 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": it is found in the serum without any evidence of prior contact with antigen.
In germline cells, the gene segment encoding the μ 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.
Two-fold structure of IgM. Left: As a two dismensional 'snowflake' and Right: As a three dismensional 'crab'.
IgM accounts for about 10% of the total antibody pool in the body and is the predominant antibody of the early immune response. IgM is the first antibody to be produced in response to infection since it does not require "class switch" to another antibody class. IgM is the only class produced by the fetus.
It is also the largest of the antibodies. IgM binds to antigens on the surface of a bacteria like a spider. Because of its shape and size, IgM is particularly good at activating complement and causing agglutination. It is the only antibody type which is capable of destroying antigen-containing cells without the assistance any other cells of the immune system. By far the greater majority of the antibodies we make to an opposing blood type (isoagglutinins) are IgM class antibodies.
IgM exists in its inert form looking much like a two-dimensional snowflake. When it attaches to a foreign cell or other antigen, it now assumes a three dimensional shape which resembles a crab. After attaching, IgM molecules can interlock, forming lattices, which cause the antigens to agglutinate. Therefore, IgM is especially well suited for the agglutination of microbes. Because of this IgM antibodies are often called ‘isoagglutinins’ which are the naturally occurring IgM antibodies against the red blood cell ABO antigens. Since they can agglutinate cells containing foreign antigens, IgM antibodies possess many of the same characteristics of the lectins.
Because the are IgM antibodies capable of agglutinating cells directly, the ‘anti-blood type antibodies’ are usually called isoagglutinins or isohemagglutinins. The power of these antibodies to agglutinate red blood cells is amply demonstrated during a blood typing session. A slide of red cells is mixed with the opposing antibody and the agglutination produced is so powerfully reactive that the results can be seen with the unaided eye.
There is evidence than non-secretors have much less IgM in their saliva than do secretors.