The immunoglobulin which is produced first by the fetus in response to infection is:

#  The immunoglobulin which is produced first by the fetus in response to infection is:

A. IgG

B. IgA

C. IgM

D. IgD

The correct answer is C. IgM.

Phylogenetically, IgM is the oldest immunoglobulin class. It is also the earliest immunoglobulin to be synthesized by the fetus beginning by about 20 weeks of age. As it is not transported across the placenta, the presence of IgM in the fetus or newborn indicates intrauterine infection and its detection is useful in the diagnosis of congenital infections such as syphilis, rubella, HIV infection and toxoplasmosis.