FluoroSpot Flex: Mouse IgM/IgA

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).

IgA (Immunoglobulin A) represents only 10-15% of the total Ig molecules in serum but is the predominating Ig isotype in secretions and in the gastrointestinal and urogenital tract. The IgA found in secretions (secretory IgA or sIgA) is commonly a dimer with two IgA molecules joined by the J chain. Secretory IgA is the most important Ig isotype in mucous surfaces, the area of entry for most pathogens.