A 26-year-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?
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 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 bih 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. IgG1 and IgG4 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 transpoed 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. Ref: Levinson W. (2012). Chapter 52. Blood & Tissue Protozoa. In W. Levinson (Ed),Review of Medical Microbiology & Immunology, 12e.
IgA
IgG 1
IgG 4
IgM