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
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| 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.
