IgM

IgM is the first immunoglobulin produced during B-cell development and the dominant class of the primary antibody response. Secreted IgM forms a pentamer assembled around a J chain, giving ten antigen-binding sites that compensate for low individual affinity and make IgM the most efficient complement-fixing antibody via the classical pathway. Surface IgM also serves as the native B-cell receptor, and naturally circulating IgM pentamers recognize apoptotic cells and self-antigens, contributing to immune homeostasis and clearance. Elevated specific IgM indicates recent infection, while IgM deficiency and hyper-IgM syndromes highlight its role in humoral immunity. Recombinant IgM formats remain an active research area.