Immunoglobulin M

Immunoglobulin M (IgM) is one of several forms of antibody that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. In the case of humans and other mammals that have been studied, the spleen, where plasmablasts responsible for antibody production reside, is the major site of specific IgM production. Immunoglobulin M (IgM) is one of several forms of antibody that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. In the case of humans and other mammals that have been studied, the spleen, where plasmablasts responsible for antibody production reside, is the major site of specific IgM production. Immunoglobulins, also known as antibodies, comprise a family of proteins that occur in five major forms, also termed classes or isotypes – IgM, IgD, IgG, IgE and IgA. Immunoglobulins are produced by vertebrate animals as part of the normal immune response to microbial, e.g., bacterial or viral, infection. Binding of the immunoglobulins to a microbe can have immediate effects, such as blocking invasion, and can mobilize other components of the immune system to destroy or otherwise inactivate the microbes, thereby providing protection against infectious diseases. The molecular structures that microbes present to the immune system and elicit immunoglobulin (antibody) production are collectively denoted as antigens. Although all vertebrates that have been studied – from fish to human – produce IgM, there are significant differences among the IgMs of different species. This article focuses on human and mouse IgM, which are both well studied and have very similar properties. For the most part IgM is produced by plasma cells in the spleen and lymph nodes and secreted into serum, where it is typically at a concentration of ~1.5 mg/mL. This article describes the structure of serum IgM, as opposed to other forms, such as the IgM membrane receptor.