The most significant pentameric immunoglobulin, immunoglobulin M (IgM), is the first antibody B cells make in response to an infection. In adults, IgM constitutes 5–10% of serum immunoglobulins. It can be found both on B lymphocytes and in the blood as a soluble molecule. It participates in opsonisation and agglutination. IgM facilitates effective immune system activation because of its numerous surface antigenic sites.
Structure of IgM
IgM lacks a hinge region in the μ-chain and has four heavyweight chain constant domains. IgM comes in two different forms:
- Monomeric IgM: It is expressed on B cells as a membrane-bound antibody.
- Pentameric IgM: Plasma cells secrete pentameric IgM. Ten antigen-binding sites are distributed around five monomer subunits, with the Fc regions located in the centre. IgM’s pentameric structure confers strong overall avidity while making up for the comparatively low affinity of IgM monomers.
The disulfide bonds and J (joining) chain that forms an interconnection in the Fc region hold the monomers together. The J chain, introduced before the pentamer secretion, facilitates the polymerisation of the monomers.
Functions of IgM
- IgM is the crucial mediator of the initial immune response and the first antibody to be produced.
- IgM is the first type of immunoglobulin produced by the newborn and is involved in developing several autoimmune disorders.
- Until enough IgG antibody has been produced in the body, the ability of IgM to combine with antigens has particular importance.
- It is essential for agglutination reactions and complement activation to fight infections because it is responsible for neutralisation, agglutination, and cytolytic reactions.
- Increased IgM levels may indicate recent infection or antigen exposure.
Related Links:
- Antibody – Role of Antibody
- Difference between T Cells and B Cells
- IgG Antibody: Structure and Function
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