Immunoglobulin M (IgM) antibodies are proteins made by the body's natural defense system (immune system). They attack and destroy things that they recognize as different from the body's normal healthy tissues, such as bacteria and viruses.

IgM antibodies are found in blood and lymph fluid and are very important in fighting infections. They are the first of the five major types of antibodies made in response to an infection, and are the largest of the antibodies. They also cause other immune system cells to fight infection.