A newborn suspected of having a congenital viral infection should be tested for virus-specific antibodies of which class(es)?

• A. IgM
• B. IgG
• C. IgA
• D. All of the above classes

Answer: (A)

When evaluating a newborn for congenital viral infection, the key distinction is which antibody classes can reveal an infant’s own immune response. IgM antibodies cannot cross the placenta, so finding virus-specific IgM in the baby means the infant has mounted an immune response to an infection occurring in utero or around birth. This makes IgM a direct indicator of congenital infection.

In contrast, IgG from the mother does cross the placenta and can be present in the newborn regardless of whether the infant is actually infected, so detecting virus-specific IgG alone cannot prove congenital infection. IgA transfer is limited and not typically used to diagnose systemic viral infections in newborns, so it’s not a reliable marker here.

Therefore, virus-specific IgM in the neonate best demonstrates a true congenital infection, whereas IgG could merely reflect maternal antibodies and IgA is not the relevant marker in this context.