Human Immunoglobulin M (IgM) is an antibody secreted by the adaptive immune system in response to foreign antigens. It is a primary type of immunoglobulin released into circulation during the early stages of the primary antibody response. Human Immunoglobulin M forms a pentamer comprising five IgG equivalents, with 10 Fab fragments, allowing for 10 antigen binding sites. Additionally, Human Immunoglobulin M acts as a complement (complement) activator.