IgM : Properties, Structure and it's Function

IgM is the membrane bound immunoglobulin. IgM is found on a surface of mature naive B-cells.

• Immunoglobulin M constitutes about 5 to 10% of the antibody concentration in the serum.

• Molecular weight of Pentameric IgM is 970 kDa. Each subunit molecular weight is 190 kDa.

• Immunoglobulin M is mainly found in a blood. It is unable to move freely because of its large size and therefore it is not found in other body fluids.

• Half-life of IgM is about 5 days.

• IgM is the first antibody that is formed by the Fetus.

• IgM is the first antibody that appeared during an immune response.

Structure of IgM

IgM occurs both in monomer and pentamer form in the body.

IgM monomer

As a monomer IgM is present on the surface of B-cells. This IgM is known as membrane-bound immunoglobulin M. 

• IgM monomer consists of 2 mue (μ) heavy chains (MW 70 kDa) and 2 light chains ( MW 23 kDa).
Each heavy chain contain 1 variable domain and 4 constant domains

Pentameric Structure of IgM

IgM  pentamer

As a pentamer IgM it is found in the secretions.

• In pentameric form 5 IgM monomers are arranged in a star-like shape.

• In this shape the antigen binding fragments are outwards and the FC fragments are towards the center. 

• These monomeric units are held together by J chain( J stands for joining and it is a glycoprotein).

• This j chain is responsible for the polymerization of IgM into pentamer. 

• Theoretically IgM pentamer has 10 antigen binding sites but only five or six of them are available for effector functions, because of steric hindrances. 

• Still this number of antigen binding sites is maximum among all antibody classes.

Functions of IgM

• IgM is the most efficient antibody which activates classical complement pathway. This is because of its large size with multiple antigen binding sites.

• A single IgM antibody can initiate the activation.

• IgM is very important defense against bacterial diseases.
  IgM antibodies are very effective neutralizers and prevent pathogens from binding to host receptors on epithelial cells.

• They do not act as opsonins needed a plan role in antibody dependent cell-mediated cytotoxicity (ADCC). This is because specific at C receptors to IgM occur rarely on the surfaces of leukocytes.