Description: Immunoglobulin M is the third most common serum Ig and can exist as pentamer where all heavy chains are identical and all light chains are identical or as a monomer (e.g. found on B lymphocytes as B - cell receptors). The large pentameric structure allows for building of bridges between encountered epitopes on molecules that are too distant as to be connected by smaller IgG antibodies. IgM is built as the first antibody during an immune response and is responsible for agglutination and cytolytic reactions since its pentameric structure gives it 10 free antigen binding sites in theory and it posseses a high avidity. Due to conformational constraints among the 10 Fab portions, IgM only has a valence of 5. Additionally, IgM is not as versatile as IgG, but it is of vital importance in complement activation and agglutination. IgM is predominantly found in the lymph fluid and the blood and is a very effective neutralizing agent in the early stages of disease. Elevated levels can be a sign of recent infection or exposure to antigen.
