What is the structure of immunoglobulin M (IgM)?

Immunoglobulin M is a glycoprotein with a molecular weight of 970 kDa. It exists in both pentameric and monomeric configurations and is composed of subunits where two heavy chains (µ) pair covalently with two light chains (L). The heavy and light chains are linked together by disulfide bonds. In addition, intrachain disulfide links divide each chain into areas known as domains.  

The µ heavy chains, which weigh approximately 72 kDa, contain about 57 amino acids arranged in five domains. 

The light chains, which weigh about 23 kDa, have two domains – one constant and one variable. 

In the pentameric configuration, 5 antibody units are linked by disulfide bonds at adjacent CH3 domains. A J-chain helps in IgM assembly and secretion. The J-chain is crucial for ensuring proper binding to poly-Ig receptors and facilitating the transport of IgM across epithelial cells.