Immunoglobulin M (IgM) is a type of antibody produced by the immune system in response to infections. It is the first antibody to be made 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 allows it to effectively bind to multiple antigens at once. This structure enhances its ability to form complexes with pathogens, facilitating their removal from the body.