Immunoglobulin M

Immunoglobulin M, or IgM (from Latin immunitas — liberation, release from something, and globulus — globule, droplet; M — the designation of the protein chain type), is the first class of antibodies that the immune system begins to produce upon initial contact with an antigen.

These are the “rapid-response forces” intended to provide emergency protection to the body until more advanced memory antibodies (IgG) are synthesized.

Etiology and pathophysiology

IgM synthesis begins 3–5 days after disease onset. In physical terms, it is the largest molecule of the immune system.

It is a pentamer — five classic Y-shaped antibodies linked in a ring. Owing to this structure, the IgM molecule comprises 10 antigen-binding sites.

IgM molecules have several specific properties:

• capacity for agglutination — although the avidity (strength) of each individual IgM interaction is lower than that of IgG, its enormous size allows it to instantly bind (agglutinate) dozens of bacteria or virions together, immobilizing them and activating the complement protein cascade to destroy them;
• inability to cross the placenta — because of its large size, IgM is physically unable to cross the placenta from the mother to the fetus.

Clinical significance

In medical practice, the applications of immunoglobulin M comprise the following aspects:

• in laboratory diagnostics — detection of immunoglobulin M is a marker of acute, active, or recent infection. During recovery, the blood IgM level rapidly decreases and disappears completely after several months;
• in neonatology — its use comprises an exceptionally important aspect. If IgM against Toxoplasma or rubella is detected in a newborn’s blood, this confirms intrauterine infection in the presence of corresponding clinical signs of congenital infection. Because maternal IgM does not cross the placenta, its presence in the infant’s blood means that the fetus was infected and its immune system produced the IgM independently.