Immunoglobulin M (IgM) is a type of antibody produced by the immune system in response to infections. It is the first antibody to be produced when the body encounters a new pathogen, helping to initiate the immune response. IgM is primarily found in the blood and lymphatic fluid, where it plays a crucial role in identifying and neutralizing foreign invaders like bacteria and viruses. IgM is unique because it exists as a pentamer, meaning five antibody units are joined together, which enhances its ability to bind to multiple antigens simultaneously. This structure allows IgM to effectively activate other components of the immune system, such as complement proteins, which help destroy pathogens and infected cells.