The immune system is the first line of defense when a disease organism or foreign substance invades the body. These invaders are called antigens, and the immune system fights the threat by developing antibodies.

The antibodies bind to the antigen molecules, triggering an immune response that will neutralize and destroy the antigen. There are five major antibody types, including immunoglobulin M (IgM). It is the largest antibody and circulates through the blood and lymphatic fluid.

B lymphocytes, or B cells, are formed in the marrow of the large bones in the body, and are responsible for the production of antibodies. There are receptors on the surface of the B cells where the antigens circulating through the body will attach. One of the functions of B cells is the production of immunoglobulins, including the IgM antibodies.

There are two types of immune responses in the body, called primary and secondary immune response. The primary response occurs when a B cell sees an antigen for the first time. Antigen binding to the surface of the B cell stimulates the production of antibodies that are capable of binding directly to the antigen. Because this first recognition process takes time for antibody development, there is an initial delay for the body to fight the invading antigens. IgM is an antibody produced during the primary immune response and plays a significant role fighting infection.

Some B cells can also change into a memory cell when exposed to an antigen for the first time. These cells will proliferate and live in the the body for a long time and can rapidly produce antibodies once it sees an antigen a second time. These memory cells circulating through the body permit someone to be immune to a disease, even if it occurs again many years later. The predominant antibody produced during a secondary immune response is immunoglobulin G (IgG).

The mechanism of the IgM production is such that the IgM molecule does not contain the highly specific binding sites of the IgG molecule. This permits the B cells to produce IgM rapidly during a primary immune response, while the IgG molecules take days to produce in quantity. The structure of the IgM molecule permits it to form a complex of five molecules, called a “pentamer." The pentamer informs the IgM function; the antibody is able to bind to many antigens simultaneously and can quickly clear antigens from the bloodstream during the initial stages of an infection.

When an antigen is introduced into the body for the first time, large quantities of IgM are produced, while the B cells are producing the highly specific IgG more slowly. Once IgG is produced in quantity, the IgG takes on a greater role in the removal of antigens from the body, due to its ability to bind to the antigen molecules more tightly. Through the course of an infection, a rapid spike of circulating IgM can be seen in the bloodstream, followed by a decrease of IgM as the amount of IgG increases. Medical personnel can identify the course and duration of an infection by measuring the ratio of IgM to IgG in the bloodstream. A ratio high in IgM indicates that an infection is in its early stages, while a ratio high in IgG indicates that the infection is in its later stage.