IgM is a type of antibody, also known as immunoglobulin M, that plays a crucial role in the immune system. It is the first antibody produced by the body in response to an infection, helping to identify and neutralize pathogens like bacteria and viruses. IgM is primarily found in the blood and lymphatic fluid. This antibody is unique because it can form a pentamer, which means five IgM molecules can join together, increasing its ability to bind to antigens. This structure allows IgM to effectively initiate the immune response and activate other components of the immune system, such as complement proteins.