- IgM is a particular isotype of Antibody (see page). IgM is the default isotype of antibody expressed by B-cells and is the only isotype synthesized prior to encounter with antigen.
- IgM can be synthesized in two different ways by B-cells: as a membrane protein or as a pentameric immunoglobulin.
- Membrane Protein
- When IgM is expressed as a membrane protein it is termed the "B-cell Receptor". In the membrane-bound form, binding of antigen to IgM can induces a variety of signal transduction cascades in the B-cell which indicate that the B-cell Receptor has encountered antigen. Consequently, expression of membrane IgM as B-cell receptor is the primary format in which B-cells express antibody during B-cell Development and the initial stages of the Humoral Immune Response.
- Pentameric Immunoglobulin
- When B-cells synthesize and secrete IgM they do so as a pentameric immunoglobulin in which the Fc Region of five IgM antibodies are covalently attached. This is done because the IgM isotype is usually the first type of antibody secreted by B-cells during a Humoral Immune Response and is thus of fairly low affinity for antigen. The low affinity of IgM is purely due to the fact that successful "Affinity Maturation" has not occurred to enhance the affinity of the antibody antigen-binding groove for the specific antigen (See: Humoral Immunity). Because of this low affinity, B-cells likely secrete IgM as a pentamer so that the antibodies can bind cooperatively to antigen, with each low-affinity interaction making the binding of adjacent IgM to the antigen more likely. Antigen-bound IgM is a highly potent at activator of Complement.
- As explained in Humoral Immunity as the humoral immune response to a pathogen matures, the isotype of antigen-specific B-cells is switched to other isotypes and thus the secretion of IgM slowly falls. Furthermore, if re-infection with the same microbe takes place, B-cells which have already undergone isotype switching to other antibody isotypes proliferate and secrete antibody; consequently, IgM isotype antibody is usually not synthesized following re-infection. Because of these properties of Humoral Immunity the presence of antigen-specific IgM titers in a patient indicates a new infection in a patient rather than a re-infection. Thus, IgM titers are often used to provide some immunological context to patients which present with signs and symptoms of an infectious disease.