Mouse immunoglobulin M (IgM) is a crucial component of the humoral immune system and is the first antibody produced by B-cells in response to an antigenic challenge. It exists in two forms: a pentameric (19S) form found in serum and a monomeric (8S) form on the surface of lymphocytes. The IgM pentamer, which is the predominant form in circulation, consists of five monomeric units and has a molecular weight of approximately 900 kDa, while the hexameric form has a molecular weight of about 1050 kDa. Each monomer of IgM is composed of two heavy chains and two light chains, with the heavy chain having four constant domains (CH1-CH4) and the light chain having one constant domain (CL). The CH4 domain facilitates oligomerization, leading to the formation of penta- or hexamers, and these oligomers are covalently linked via disulfide bonds, resembling a snowflake structure. A joining (J) chain, a 15 kDa polypeptide, is essential for the secretion of IgM into mucosal areas.