IgM antibodies

Immunoglobulin M (IgM) is an antibody produced by B cells. IgM is the largest antibody in the human circulatory system. It is the first antibody to appear in response to initial exposure to an antigen. 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 the microbes can mobilize other components of the immune system to destroy or otherwise inactivate the microbes and thereby provide protection against infectious disease. The molecular structures that bacteria and viruses 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 in the IgM 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.

Immunoglobulins are composed of two types of protein chain -- the heavy chain and the light chain. The heavy chains, which determine the immunoglobulin class, are of five different types, denoted by the Greek letters, µ, δ, γ, ε and α, corresponding to the classes, IgM, IgD, IgG, IgE, and IgA, respectively. Light chains are of two types, denoted λ and κ. Each chain is itself divided into two functional parts, the variable (V) domain and the constant (C) domain. The V domains of the light and heavy chain are juxtaposed to form a structure that binds antigen; different V domains bind different antigens, i.e., binding of antigen by immunoglobulin is “antigen-specific”. Inasmuch as the V domains can occur in a nearly unlimited variety of amino acid sequences, immunoglobulins collectively have the potential of binding to virtually any molecular structure. The immunoglobulin C domains interact with other physiological components, eg the complement system. Thus, the constant domain can mobilize the complement system to act on the antigen that is bound by the variable domain.

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