The "big", multivalent antibody , capable of capturing and binding antigens to form large insoluble complexes which are then readily cleared from the blood. Usually the first antibody to appear after initial exposure to an antigen. The isohemaglutin (anti-A, anti-B) rheumatoid factors as well as antibodies to micro-organisms are IgM. IgM levels may be elevated in patients with delayed patterns of food allergy and probably manifest a protective defense response.

IgM has a characteristic pentameric structure and is largely confined to the blood. It comprises 5-10% of the body's total immunoglobulin.