IgGIgM

IgG and IgM are two major immunoglobulin classes produced by B cells that play essential roles in the immune response. IgG is the most abundant antibody in blood and extracellular fluids, providing long-term protection and shaping the quality of the antibody response. IgM is the first antibody isotype produced during a primary immune response and is particularly important for early defense against infection.

IgG is a monomer composed of two gamma heavy chains and two light chains, and it comprises four subclasses (IgG1–IgG4) that differ in Fc region properties, influencing receptor binding and effector functions. IgM is a pentamer in most circulating forms, formed by five monomer units linked by a joining (J) chain, which gives it high overall valency and strong avidity. The pentameric structure makes IgM especially effective at agglutination and at activating the classical complement pathway.

B cells initially express membrane-bound IgM as a receptor (IgM BCR). Upon activation, they may undergo class switch recombination to produce other isotypes such as IgG, IgA, or IgE, enabling different effector functions while preserving antigen specificity. In adults, IgG is actively transported across the placenta by the neonatal Fc receptor (FcRn), providing passive fetal immunity. IgM does not cross the placenta.

Clinically, the presence of IgM often indicates recent exposure or an acute infection, while IgG tends to reflect past exposure or longer-term immunity. IgG subclasses contribute to diverse effector mechanisms including opsonization and Fc receptor engagement, whereas IgM is particularly effective at initiating complement activation and rapid pathogen agglutination.