Anti‑mouse IgM mAb (MT9A2), biotin

Immunoglobulin M (IgM) is the first antibody isotype (also called class) secreted by the adaptive immune system in response to a foreign antigen. Plasma B cells secrete IgM mainly as pentamers, i.e., five IgM molecules joined together by the J chain. Because IgM is formed early in the immune response and is later replaced by IgG, specific antibodies of the IgM class are characteristic of a recent, ongoing, or chronic infection.  Therefore, IgM represents the isotype with the lowest affinity for antigens. Due to its structure, with 10 antigen-binding sites per IgM pentamer, it can nevertheless bind surface-bound antigens with very high avidity. Immunoglobulin M constitutes approximately 10% of the total immunoglobulin concentration in serum. Resting mature yet naive, B lymphocytes express monomeric IgM as a transmembrane antigen receptor that functions as part of the B cell receptor (BCR).