Final answer:
IgM, standing for 'Macroglobulin', is an antibody that forms a pentamer structure with the ability to bind up to ten identical antigens. It is the first antibody produced in the immune response, essential for early defense against pathogens, and serves as a diagnostic marker for infections.
Step-by-step explanation:
Properties of IgM:
The 'M' in IgM stands for 'Macroglobulin', which indicates its relatively large size compared to other classes of antibodies. IgM molecules are composed of five monomers, each being a four-chain structure, linked together by a joining (J) chain to form a pentamer. This pentamer arrangement allows IgM to bind up to ten identical antigens effectively. IgM plays a significant role as the first antibody produced during the primary immune response, and given its structure, it can initiate early immune actions against pathogens. Despite its effectiveness in early immune responses, IgM does not bind to antigens as stably as IgG, the predominant antibody during secondary and subsequent exposures to antigens.
Naïve B cells are predominantly of the IgM class. When they encounter antigens, these B cells can differentiate into plasma cells that secrete IgM. IgM can also act as an antigen receptor on the surface of B cells. Secreted IgM, due to its pentameric structure and ten Fab antigen-binding sites, is crucial in the body's initial defense against pathogens and is used as a marker for active or recent infections.