Paper I
Question
lgM.
Answer
IgM is the largest immunoglobulin, existing as a pentamer (five monomeric units joined by a J chain), giving it 10 antigen-binding sites (though functional valency is often lower due to steric constraints) and a molecular weight of ~900,000 Da — too large to cross the placenta.
Key features:
- First antibody produced in a primary immune response, appearing early (within days) and declining as IgG production rises — its presence in serum therefore indicates recent/current infection.
- Because it does not cross the placenta, IgM in a neonate’s serum indicates intrauterine (congenital) infection — used diagnostically in TORCH infections.
- Most efficient antibody class for complement activation (classical pathway) and for agglutination/cytolysis, owing to its multivalency — the 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) but is the first class secreted in a primary response before class-switching to IgG.