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Originally Posted by Oak A 28 yrs old obstetric patient becomes acutely ill during her first trimester with infectious mononucleosis-like symptoms, but her heterophil antibody test was negative. A careful history reveals that the family has two cats in the house. The appropriate laboratory tests indicate the expectant mother is infected with Toxoplasma gondii. Months later, the woman delivers a full term baby with no obvious signs of infection with the protozoan parasite. The best test to diagnose acute infection in the neonate would be a parasite-specific ELISA for which isotype of immunoglobulin? A. IgA
B. IgE
C. IgG 1
D. IgG 4
E. IgM
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The correct answer is E. IgM immunoglobulin directed against Toxoplasma would provide evidence of infection in the newborn baby. IgM is the only antibody that a baby can form with an acute infection. It is the first antibody that is formed in an infection and it is also the antibody
that is present on the surface of immature and mature B cells.
IgA (choice A) is the antibody that is produced in response to mucosal infections. It cannot cross the placenta. The baby's immune system is still not well developed at birth and cannot produce IgA immunoglobulin. In adults or older children, IgA is exists in serum in a monomeric form but is present
in seromucous secretions as a dimer.
IgE (choice B) is the antibody that is produced in response to an allergen. It cannot cross the placenta, and is not present in the baby at birth or for some time after birth. The baby's immune system is still not well developed at birth and can initially only form IgM immunoglobulin.
IgG 1 (choice C) and IgG4 (choice D) immunoglobulins might be present in the baby, but would not indicate infection in the infant. The presence of these isotypes would indicate that the mother was infected with the organism and produced antibody that was transported across the placenta in utero. IgG is the only immunoglobulin that can cross the placenta, providing protection to the baby during the first few months of life.