IgM stands for 'Immunogobulin M'. It is an antibody that is mainly concentrated in the blood and is the first to act in the primary antibody response to a pathogen/antigen. IgM binds tightly to antigens containing multiple identical epitopes due to the presence of 10 combining sites. In proteins, avidity describes the strength of multiple bonds whereas affinity denotes the binding of a single site.
IgM is a pentamer, making it the largest of the immunoglobulins, having a MW of 900 kDa. This is covalently bonded to the J chain which is involved in polymerisation and has a MW of 15 kDa. It has a theoretical valency of 10, but only 5 in practice. Usually it has a high avidity and a low affinity.