The plasma cell produces immunoglobulins (or antibodies) as an immune response to an antigen, i.e. non-self agent. An immunoglobulin (Ig) is a glycoprotein with a Y-shape. The basic structure of a monomeric unit of antibody consists of two identical heavy chains and two identical light chains. In each of these chains, there are variable and constant regions. The immunoglobulin heavy chain, in particular, is the large polypeptide subunit. It can be used to classify the various isotypes (or classes) of immunoglobulins. In humans and other mammals, the five classes are: (1) immunoglobulin A (IgA), (2) immunoglobulin D (IgD), (3) immunoglobulin E (IgE), (4) immunoglobulin G (IgG), and (5) immunoglobulin M (IgM). The immunoglobulin M (IgM) has μ (mu)-type of immunoglobulin heavy chain (thus, the acronym M). The light chain of IgM may either be kappa (κ) or lambda (λ). Its molecular formula is (μ2 κ2)5 or (μ2λ2)5.
In humans, IgM may occur as a monomer or as a cyclic pentamer. A monomeric IgM is expressed on the surface of B cells. A pentameric IgM is the form of IgM found in secretions. A pentamer would mean there are five functional units of antibodies joined by J chains. In its pentameric form, it is the largest antibody, with a molecular weight of 970 kD. And because of being a pentamer, IgM is a strong complement activator and agglutinator.
IgM is regarded as the primary response antibody because it is typically the first antibody to be produced (even before IgG) in the initial exposure to an antigen. Thus, the IgMs are the first antibodies to provide protection against microbial infections. They are typically the antibodies that prevail during the early stages of humoral immunity. The spleen is the major site of IgM production.