Question
Enumerate the classes of immunoglobulins. Write briefly on structure and properties of IgM
Answer
Classes of immunoglobulins: IgG, IgA, IgM, IgD, IgE.
Structure and properties of IgM Structure: the largest immunoglobulin, existing as a pentamer — five monomeric units, each with the basic four-chain structure (two heavy μ chains, two light chains), joined together by a J (joining) chain, giving it up to 10 potential antigen-binding sites (functional valency often lower due to steric constraints) and a molecular weight of ~900,000 Da.
Properties:
- Too large to cross the placenta — unlike IgG.
- First antibody produced in a primary immune response, appearing early (within days) and declining as IgG production rises — its presence in serum indicates recent/current infection.
- Because it does not cross the placenta, IgM in a neonate’s serum indicates intrauterine (congenital) infection — the basis of TORCH panel interpretation.
- Most efficient antibody class for complement activation (classical pathway) and agglutination/cytolysis, owing to its multivalency — natural isohaemagglutinins (anti-A, anti-B) of the ABO blood group system are IgM.
- Confined mainly to the intravascular compartment due to its large size.
- Serves as the B-cell receptor (membrane-bound monomeric form) on naive B lymphocytes.
- Present in low concentration in normal serum (~5–10% of total immunoglobulin), being the first class secreted in a primary response before class-switching to IgG.
Clinical significance: IgM serology (e.g., IgM-based rapid tests for dengue, hepatitis A, leptospirosis) is valuable for diagnosing acute infection; Waldenström’s macroglobulinaemia is a lymphoproliferative disorder of monoclonal IgM overproduction, causing hyperviscosity syndrome.