FluoroSpot Flex: Mouse IgM/IgG

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).

Immunoglobulin G (IgG) is the most abundant Ig isotype in serum, making up approximately 80% of all serum immunoglobulins. In humans, there are four subclasses of immunoglobulin G, with the highest serum concentrations of IgG1 followed by IgG2, IgG3, and IgG4. In mice, the IgG subclasses are defined as IgG1, IgG2a/c, IgG2b, and IgG3. The IgG molecule consists of two heavy and two light chains (κ or λ), resulting in a molecule with two arms for antigen binding. High levels of IgG antibodies are induced following the initial IgM response in a typical immune response to antigens.